[RFA:] fix lack of #ifdef HAVE_C99_RUNTIME in builtins-20.c and elsewhere

[Geoff Keating]

> +proc check_effective_target_c99_math { } {
> +    global srcdir
> +    return [check_no_compiler_messages c99_math assembly {
> +	#include "builtins-config.h"
> +	#ifndef HAVE_C99_RUNTIME
> +	#error "No HAVE_C99_RUNTIME"
> +	#endif
> +    } -I$srcdir/gcc.dg ]
> +}

This won't work when running the testsuite outside the build tree.   
Instead you need to either install the header file or have the  
compiler #define something with underscores (eg.  
__HAVE_C99_RUNTIME__).  Patch is not OK.