bug#32924: 27.0.50; Error building master on macOS Mojave

[Paul Eggert <eggert@cs.ucla.edu>]

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> $ gcc --version
> Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1
> Apple LLVM version 10.0.0 (clang-1000.11.45.2)

Wait, the command 'gcc' on macOS actually runs Clang? That's cold. How do you 
think Apple would feel if we called Emacs "iPhone"?

Anyway, we can pacify Clang by disabling its -Wnull-pointer-arithmetic option, 
so I installed the attached. I'd rather do this than go back to the old intptr_t 
way of doing things, which has too many problems with missing type-checking at 
the C language level (unless you configure with --enable-check-lisp-object-type, 
which has its own set of problems).

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From 7f1beabfcdcb58d90aa78db22b9a123faf298749 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Tue, 9 Oct 2018 11:16:00 -0700
Subject: [PATCH] Port --enable-gcc-warnings to recent clang

* configure.ac: Disable -Wnull-pointer-arithmetic if clang (Bug#32924).
---
 configure.ac | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 6f3d7338c3..df910280b4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1019,9 +1019,10 @@ AC_DEFUN
   gl_WARN_ADD([-Wno-unused-parameter]) # Too many warnings for now
   gl_WARN_ADD([-Wno-format-nonliteral])
 
-  # clang is unduly picky about braces.
+  # clang is unduly picky about some things.
   if test "$emacs_cv_clang" = yes; then
     gl_WARN_ADD([-Wno-missing-braces])
+    gl_WARN_ADD([-Wno-null-pointer-arithmetic])
   fi
 
   # This causes too much noise in the MinGW build
-- 
2.17.1