Emacs 28.1 doesn't compile on Mac OS 10.7.5

[Werner LEMBERG]

Compiling emacs 28.1 fails if built with MacPorts on Mac OS 10.7.5
(Lion).  The error messages start with

```
/usr/bin/clang -c -isystem/opt/local/include/LegacySupport \
  -I/opt/local/include -Demacs -I. -I. -I../lib -I../lib \
  -I/opt/local/include/libxml2 -MMD -MF deps/coding.d -MP \
  -I/opt/local/include/p11-kit-1 -Wno-switch -Wno-pointer-sign \
  -Wno-string-plus-int -Wno-initializer-overrides \
  -Wno-tautological-compare -pipe -Os -arch x86_64
  coding.c
coding.c:2632:10: error: type name requires a specifier or qualifier
                code = ENCODE_CHAR (charset, c);
                       ^
./charset.h:432:4: note: expanded from macro 'ENCODE_CHAR'
  (verify_expr                                                          \
   ^
../lib/verify.h:262:5: note: expanded from macro 'verify_expr'
   (_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
    ^
../lib/verify.h:177:15: note: expanded from macro '_GL_VERIFY_TRUE'
   (!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
              ^
../lib/verify.h:192:7: note: expanded from macro '_GL_VERIFY_TYPE'
      _Static_assert (R, DIAGNOSTIC);          \
      ^
```

A complete failure log can be found at

  
https://build.macports.org/builders/ports-10.7_x86_64-builder/builds/95632/steps/install-port/logs/stdio

Note that `/usr/bin/clang -v` on this ancient Mac gives the following
output.

```
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
```


    Werner