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.

(cherry picked from commit cbfba7a0e9)

testsc.c

@@ -81,7 +81,7 @@
 #include "mpint.h"
 #include "ecc.h"
 
-static NORETURN void fatal_error(const char *p, ...)
+static NORETURN PRINTF_LIKE(1, 2) void fatal_error(const char *p, ...)
 {
     va_list ap;
     fprintf(stderr, "testsc: ");