Check explicitly for clang++ usage

gcc might also be used on linux instead of clang++ but the
current implementation would still include these clang++ specific
flags which are not supported by gcc.

Change-Id: I1345bff16d9bf07f1c81506fef4749963182f299
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/13900
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index 6738f44..ce600e7 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -446,8 +446,8 @@
         '-std=c++17'
     ])
 
-    # flag not supported by gcc/g++.
-    if not (platform.is_aix() or platform.is_msys() or platform.is_mingw()):
+    # flags not supported by gcc/g++.
+    if cxx == 'clang++':
       cflags.extend(['-Wrange-loop-analysis', '-Wextra-semi-stmt'])
 
     if platform.is_linux() or platform.is_mingw() or platform.is_msys():