include: Simplify glibc version dependent macro handling

We test once whether __GLIBC__ is not defined, so we do not need to test
whether it is on the OR branch afterwards. We decouple the glibc version
restriction check from the _*_SOURCE variable, as that contains an
implicit opposite version check.

include/bsd/stdlib.h

@@ -72,7 +72,8 @@ int sradixsort(const unsigned char **base, int nmemb,
 
 void *reallocf(void *ptr, size_t size);
 #if !defined(__GLIBC__) || \
-    (defined(__GLIBC__) && (!__GLIBC_PREREQ(2, 26) || !defined(_GNU_SOURCE)))
+    !__GLIBC_PREREQ(2, 26) || \
+    !defined(_GNU_SOURCE)
 void *reallocarray(void *ptr, size_t nmemb, size_t size);
 #endif
 void *recallocarray(void *ptr, size_t oldnmemb, size_t nmemb, size_t size);