Guard individual compatibility header prototypes.

This is to avoid redefining prototypes from the libc headers. Also,
simplify the autoconf function checks and remove some copy/paste
errors checking for 'write'.

ok wouter@

include/stdlib.h

@@ -7,10 +7,18 @@
 #include <sys/time.h>
 #include <stdint.h>
 
+#ifdef NO_ARC4RANDOM_BUF
 uint32_t arc4random(void);
 void arc4random_buf(void *_buf, size_t n);
+#endif
+
+#ifdef NO_REALLOCARRAY
 void *reallocarray(void *, size_t, size_t);
+#endif
+
+#ifdef NO_STRTONUM
 long long strtonum(const char *nptr, long long minval,
 		long long maxval, const char **errstr);
+#endif
 
 #endif