warnings.m4: provide a means to specify the program to compile.

* m4/warnings.m4 (gl_COMPILER_OPTION_IF): New, extracted from
(gl_WARN_ADD): here.
Use gl_AS_VAR_APPEND.
Support an argument to specify the program to compile.
(gl_WARN_ADD): Accept an argument to specify the program to compile.
AC_SUBST the WARN_CFLAGS when they are used.
* modules/warnings (configure.ac): Don't AC_SUBST WARN_CFLAGS,
leave this to gl_WARN_ADD.