switch from c17 to gnu99

I am choosing gnu99 over c99, since I am planning to use GNU extensions,
like bswap. (in a future commit)
Why I choose c99 over c17, is because this'll produce more portable
code.

c99 does not implement noreturn.h, thus I added it to the compiler
attributes header.

src/util/atrb.h

@@ -8,6 +8,7 @@
 #define atrb_unused
 #define atrb_pure
 #define atrb_const
+#define atrb_noreturn
 #define atrb_format()
 #define atrb_nonnull()
 
@@ -34,6 +35,11 @@
 #define atrb_const __attribute__((const))
 #endif
 
+#if __has_attribute(noreturn)
+#undef atrb_noreturn
+#define atrb_noreturn __attribute__((noreturn))
+#endif
+
 #if __has_attribute(format)
 #undef atrb_format
 #define atrb_format(...) __attribute__((format(__VA_ARGS__)))