Cleanup GCC compiler attributes

Added define 'unused_' for '__attribute__((unused))'. The oldest version
of GCC with online docs (GCC 2.95.3, released in March 16, 2001 [1])
already has support for this attribute, so it doesn't make sense to
check the version.

Renamed 'noreturn' to 'noreturn_' for consistency.

[1] https://gcc.gnu.org/onlinedocs/

Signed-off-by: Antonio Niño Díaz <[email protected]>

include/link/script.h

@@ -13,7 +13,7 @@
 
 #include "helpers.h"
 
-noreturn void script_fatalerror(const char *fmt, ...);
+noreturn_ void script_fatalerror(const char *fmt, ...);
 
 void script_Parse(const char *path);