build: On macOS do not build functions provided by the system

We have never built before on macOS, so we can exclude all the functions
that are currently provided in the system.

Closes: #1
Closes: !3

include/bsd/string.h

@@ -41,10 +41,14 @@
 #include <sys/types.h>
 
 __BEGIN_DECLS
+#ifndef __APPLE__
 size_t strlcpy(char *dst, const char *src, size_t siz);
 size_t strlcat(char *dst, const char *src, size_t siz);
+#endif
 char *strnstr(const char *str, const char *find, size_t str_len);
+#ifndef __APPLE__
 void strmode(mode_t mode, char *str);
+#endif
 
 #if !defined(__GLIBC__) || \
     !__GLIBC_PREREQ(2, 25) || \