My first thought is that you need to install the xcode command line tools.
You do something like:

    xcode-select --install

(I'm using my phone, so I hope this is formatted reasonably.)

On 12 Oct 2017 7:20 a.m., "George Plymale II" <georgie@southernohio.net>
wrote:

So I tried compiling Emacs 26 on macOS 10.13. I configured with
`./configure' and everything went fine. However, when I ran `make', the
following error appeared:



/Applications/Xcode.app/Contents/Developer/usr/bin/make -C lib all
  CC       acl_entries.o
  CC       explicit_bzero.o
  CC       fpending.o
  CC       fstatat.o
  CC       getopt.o
  CC       getopt1.o
  CC       lstat.o
  CC       memrchr.o
  CC       mktime.o
  CC       open.o
open.c:75:27: error: unknown type name 'PROMOTED_MODE_T'
      mode = va_arg (arg, PROMOTED_MODE_T);
                          ^
1 error generated.
make[1]: *** [open.o] Error 1
make: *** [lib] Error 2



Upon inspecting this error, I see the following comment above the
offending code:

/* We have to use PROMOTED_MODE_T instead of mode_t, otherwise GCC 4
   creates crashing code when 'mode_t' is smaller than 'int'.  */

I also see the following output regarding `open.c' when I compile with
`make CFLAGS="-v"':



...
  CC       open.o
Apple LLVM version 9.0.0 (clang-900.0.37)
Target: x86_64-apple-darwin17.0.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/
XcodeDefault.xctoolchain/usr/bin
 "/Applications/Xcode.app/Contents/Developer/Toolchains/
XcodeDefault.xctoolchain/usr/bin/clang" -cc1 -triple
x86_64-apple-macosx10.13.0 -Wdeprecated-objc-isa-usage
-Werror=deprecated-objc-isa-usage -emit-obj -mrelax-all -disable-free
-disable-llvm-verifier -discard-value-names -main-file-name open.c
-mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim
-fno-strict-return -masm-verbose -munwind-tables -target-cpu penryn
-target-linker-version 302.3 -v -dwarf-column-info -debugger-tuning=lldb
-coverage-notes-file /Users/my_username/path/to/emacs/lib/open.gcno
-resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/
XcodeDefault.xctoolchain/usr/lib/clang/9.0.0 -dependency-file deps/open.d
-MT open.o -MP -I . -I ../src -I . -I ./../src -Wno-switch
-Wno-pointer-sign -Wno-string-plus-int -Wno-unknown-attributes
-Wno-initializer-overrides -Wno-tautological-compare
-Wno-tautological-constant-out-of-range-compare -fdebug-compilation-dir
/Users/my_username/path/to/emacs/lib -ferror-limit 19 -fmessage-length 119
-stack-protector 1 -fblocks -fobjc-runtime=macosx-10.13.0
-fencode-extended-block-signature -fmax-type-align=16
-fdiagnostics-show-option -fcolor-diagnostics -o open.o -x c open.c
clang -cc1 version 9.0.0 (clang-900.0.37) default target
x86_64-apple-darwin17.0.0
ignoring duplicate directory "."
ignoring duplicate directory "../src"
#include "..." search starts here:
#include <...> search starts here:
 .
 ../src
 /usr/local/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/
XcodeDefault.xctoolchain/usr/lib/clang/9.0.0/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/
XcodeDefault.xctoolchain/usr/include
 /usr/include
 /System/Library/Frameworks (framework directory)
 /Library/Frameworks (framework directory)
End of search list.
open.c:75:27: error: unknown type name 'PROMOTED_MODE_T'
      mode = va_arg (arg, PROMOTED_MODE_T);
                          ^
1 error generated.
make[1]: *** [open.o] Error 1
make: *** [lib] Error 2



Anyone know how to fix this error? I'd hate to run into something like
this in a non-release-candidate release...