Fix build with gcc12

Need to skip a bogus flag on gcc 12, reported here:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104336

Change-Id: I5b3fd530d20902247d4ea9b5285bf0a389e2fa45
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/16820
Commit-Queue: Brett Wilson <brettw@google.com>
Reviewed-by: David Turner <digit@google.com>
Reviewed-by: Brett Wilson <brettw@google.com>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 3570746..169427e 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -229,6 +229,17 @@
   return 0
 
 
+def is_gcc(cxx):
+  """Return True iff the compiler at `cxx` is GCC based."""
+  ret = subprocess.run(
+      f'{cxx} -dM -E -',
+      shell=True,
+      stdin=subprocess.DEVNULL,
+      text=True,
+      capture_output=True)
+
+  return ret.returncode == 0 and "#define __GNUC__" in ret.stdout and not "#define __clang__" in ret.stdout
+
 def GenerateLastCommitPosition(host, header):
   ROOT_TAG = 'initial-commit'
   describe_output = subprocess.check_output(
@@ -480,11 +491,13 @@
         '-std=c++20'
     ])
 
-    # flags not supported by gcc/g++.
-    if cxx == 'clang++':
-      cflags.extend(['-Wrange-loop-analysis', '-Wextra-semi-stmt'])
-    else:
+    if is_gcc(cxx):
       cflags.append('-Wno-redundant-move')
+      # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104336
+      cflags.append('-Wno-restrict')
+    # flags not supported by gcc/g++.
+    else:
+      cflags.extend(['-Wrange-loop-analysis', '-Wextra-semi-stmt'])
 
     if platform.is_linux() or platform.is_mingw() or platform.is_msys():
       ldflags.append('-Wl,--as-needed')