Fixes non __GNUC__ compilation

adds missing check for defined(__GNUC__)
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1094)

include/openssl/e_os2.h

@@ -293,7 +293,7 @@ typedef unsigned __int64 uint64_t;
 #  define ossl_inline inline
 # endif
 
-# if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7)
+# if defined(__GNUC__) && (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 7))
 #  define ossl_noreturn __attribute__((noreturn))
 # elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
 #  define ossl_noreturn _Noreturn