This patch replaces the OS-specific detection mechanism by one that solely
relies on environment variables.  This has the benefit that said environment
variables can be used as search paths in Guix.

Index: zig-0.8.1/lib/std/zig/system.zig
===================================================================
--- zig-0.8.1.orig/lib/std/zig/system.zig
+++ zig-0.8.1/lib/std/zig/system.zig
@@ -39,101 +39,57 @@ pub const NativePaths = struct {
         };
         errdefer self.deinit();
 
-        var is_nix = false;
-        if (process.getEnvVarOwned(allocator, "NIX_CFLAGS_COMPILE")) |nix_cflags_compile| {
-            defer allocator.free(nix_cflags_compile);
-
-            is_nix = true;
-            var it = mem.tokenize(nix_cflags_compile, " ");
+        // TODO: Support cross-compile paths?
+        if (process.getEnvVarOwned(allocator, "C_INCLUDE_PATH")) |c_include_path| {
+            defer allocator.free(c_include_path);
+            var it = mem.tokenize(c_include_path, ":");
             while (true) {
-                const word = it.next() orelse break;
-                if (mem.eql(u8, word, "-isystem")) {
-                    const include_path = it.next() orelse {
-                        try self.addWarning("Expected argument after -isystem in NIX_CFLAGS_COMPILE");
-                        break;
-                    };
-                    try self.addIncludeDir(include_path);
-                } else {
-                    if (mem.startsWith(u8, word, "-frandom-seed=")) {
-                        continue;
-                    }
-                    try self.addWarningFmt("Unrecognized C flag from NIX_CFLAGS_COMPILE: {s}", .{word});
-                }
+                const dir = it.next() orelse break;
+                try self.addIncludeDir(dir);
             }
         } else |err| switch (err) {
             error.InvalidUtf8 => {},
             error.EnvironmentVariableNotFound => {},
             error.OutOfMemory => |e| return e,
         }
-        if (process.getEnvVarOwned(allocator, "NIX_LDFLAGS")) |nix_ldflags| {
-            defer allocator.free(nix_ldflags);
 
-            is_nix = true;
-            var it = mem.tokenize(nix_ldflags, " ");
+        if (process.getEnvVarOwned(allocator, "CPLUS_INCLUDE_PATH")) |cplus_include_path| {
+            defer allocator.free(cplus_include_path);
+            var it = mem.tokenize(cplus_include_path, ":");
             while (true) {
-                const word = it.next() orelse break;
-                if (mem.eql(u8, word, "-rpath")) {
-                    const rpath = it.next() orelse {
-                        try self.addWarning("Expected argument after -rpath in NIX_LDFLAGS");
-                        break;
-                    };
-                    try self.addRPath(rpath);
-                } else if (word.len > 2 and word[0] == '-' and word[1] == 'L') {
-                    const lib_path = word[2..];
-                    try self.addLibDir(lib_path);
-                } else {
-                    try self.addWarningFmt("Unrecognized C flag from NIX_LDFLAGS: {s}", .{word});
-                    break;
-                }
+                const dir = it.next() orelse break;
+                try self.addIncludeDir(dir);
             }
         } else |err| switch (err) {
             error.InvalidUtf8 => {},
             error.EnvironmentVariableNotFound => {},
             error.OutOfMemory => |e| return e,
         }
-        if (is_nix) {
-            return self;
-        }
-
-        if (comptime Target.current.isDarwin()) {
-            try self.addIncludeDir("/usr/include");
-            try self.addIncludeDir("/usr/local/include");
 
-            try self.addLibDir("/usr/lib");
-            try self.addLibDir("/usr/local/lib");
-
-            try self.addFrameworkDir("/Library/Frameworks");
-            try self.addFrameworkDir("/System/Library/Frameworks");
-
-            return self;
+        if (process.getEnvVarOwned(allocator, "LIBRARY_PATH")) |library_path| {
+            defer allocator.free(library_path);
+            var it = mem.tokenize(library_path, ":");
+            while (true) {
+                const dir = it.next() orelse break;
+                try self.addLibDir(dir);
+            }
+        } else |err| switch (err) {
+            error.InvalidUtf8 => {},
+            error.EnvironmentVariableNotFound => {},
+            error.OutOfMemory => |e| return e,
         }
 
-        if (native_target.os.tag != .windows) {
-            const triple = try native_target.linuxTriple(allocator);
-            const qual = native_target.cpu.arch.ptrBitWidth();
-
-            // TODO: $ ld --verbose | grep SEARCH_DIR
-            // the output contains some paths that end with lib64, maybe include them too?
-            // TODO: what is the best possible order of things?
-            // TODO: some of these are suspect and should only be added on some systems. audit needed.
-
-            try self.addIncludeDir("/usr/local/include");
-            try self.addLibDirFmt("/usr/local/lib{d}", .{qual});
-            try self.addLibDir("/usr/local/lib");
-
-            try self.addIncludeDirFmt("/usr/include/{s}", .{triple});
-            try self.addLibDirFmt("/usr/lib/{s}", .{triple});
-
-            try self.addIncludeDir("/usr/include");
-            try self.addLibDirFmt("/lib{d}", .{qual});
-            try self.addLibDir("/lib");
-            try self.addLibDirFmt("/usr/lib{d}", .{qual});
-            try self.addLibDir("/usr/lib");
-
-            // example: on a 64-bit debian-based linux distro, with zlib installed from apt:
-            // zlib.h is in /usr/include (added above)
-            // libz.so.1 is in /lib/x86_64-linux-gnu (added here)
-            try self.addLibDirFmt("/lib/{s}", .{triple});
+        if (process.getEnvVarOwned(allocator, "DYLD_FRAMEWORK_PATH")) |dyld_framework_path| {
+            defer allocator.free(dyld_framework_path);
+            var it = mem.tokenize(dyld_framework_path, ":");
+            while (true) {
+                const dir = it.next() orelse break;
+                try self.addFrameworkDir(dir);
+            }
+        } else |err| switch (err) {
+            error.InvalidUtf8 => {},
+            error.EnvironmentVariableNotFound => {},
+            error.OutOfMemory => |e| return e,
         }
 
         return self;