BUILD: only enable -Wno-vla-cxx-extension if clang++ supports it

Signed-off-by: Stefan Westerfeld <[email protected]>

Author: swesterfeld

configure.ac

@@ -463,8 +463,7 @@ MC_PROG_CC_SUPPORTS_OPTION([-Wall], [
 ])
 
 if test "x$compiler_is_clang" = "xyes"; then
-  AM_CFLAGS="$AM_CFLAGS -Wno-vla-cxx-extension"
-  AM_CXXFLAGS="$AM_CXXFLAGS -Wno-vla-cxx-extension"
+  MC_PROG_CC_SUPPORTS_OPTION([-Wvla-cxx-extension -Werror], [ AM_CXXFLAGS="$AM_CXXFLAGS -Wno-vla-cxx-extension" ])
   WARN_GLOBAL_CDTORS="-Wglobal-constructors -Wexit-time-destructors"
 else
   WARN_GLOBAL_CDTORS=""