Fix function declaration protection for glibc already providing them

On non-glibc based systems we cannot unconditionally use the
__GLIBC_PREREQ macro as it gets expanded before evaluation. Instead,
if it is undefined, define it to 0.

We should also always declare these functions on non-glibc based
systems. And on systems with a new enough glibc, which provides these
functions, we should still provide the declarations if _GNU_SOURCE
is *not* defined.

Reported-by: Jörg Krause <joerg.krause@embedded.rocks>

include/bsd/sys/cdefs.h

@@ -58,6 +58,14 @@
 #endif
 #endif
 
+/*
+ * On non-glibc based systems, we cannot unconditionally use the
+ * __GLIBC_PREREQ macro as it gets expanded before evaluation.
+ */
+#ifndef __GLIBC_PREREQ
+#define __GLIBC_PREREQ(maj, min) 0
+#endif
+
 /*
  * Some kFreeBSD headers expect those macros to be set for sanity checks.
  */