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/string.h

@@ -12,21 +12,34 @@
 #include <strings.h>
 #endif
 
+#ifdef NO_STRLCPY
 size_t strlcpy(char *dst, const char *src, size_t siz);
+#endif
 
+#ifdef NO_STRLCAT
 size_t strlcat(char *dst, const char *src, size_t siz);
+#endif
 
+#ifdef NO_STRNDUP
 char * strndup(const char *str, size_t maxlen);
-
 size_t strnlen(const char *str, size_t maxlen);
+#endif
 
+#ifdef NO_EXPLICIT_BZERO
 void explicit_bzero(void *, size_t);
+#endif
 
+#ifdef NO_TIMINGSAFE_BCMP
 int timingsafe_bcmp(const void *b1, const void *b2, size_t n);
+#endif
 
+#ifdef NO_TIMINGSAFE_MEMCMP
 int timingsafe_memcmp(const void *b1, const void *b2, size_t len);
+#endif
 
+#ifdef NO_MEMMEM
 void * memmem(const void *big, size_t big_len, const void *little,
 	size_t little_len);
+#endif
 
 #endif