Greatly improve printf format-string checking.

I've added the gcc-style attribute("printf") to a lot of printf-shaped
functions in this code base that didn't have it. To make that easier,
I moved the wrapping macro into defs.h, and also enabled it if we
detect the __clang__ macro as well as __GNU__ (hence, it will be used
when building for Windows using clang-cl).

The result is that a great many format strings in the code are now
checked by the compiler, where they were previously not. This causes
build failures, which I'll fix in the next commit.

cmdgen.c

@@ -1205,7 +1205,7 @@ void test(int retval, ...)
     sfree(argv);
 }
 
-void filecmp(char *file1, char *file2, char *fmt, ...)
+PRINTF_LIKE(3, 4) void filecmp(char *file1, char *file2, char *fmt, ...)
 {
     /*
      * Ideally I should do file comparison myself, to maximise the
@@ -1303,7 +1303,7 @@ char *get_fp(char *filename, unsigned flags)
     return toret;
 }
 
-void check_fp(char *filename, char *fp, char *fmt, ...)
+PRINTF_LIKE(3, 4) void check_fp(char *filename, char *fp, char *fmt, ...)
 {
     char *newfp;