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/grp.h

@@ -44,8 +44,10 @@
 __BEGIN_DECLS
 int
 gid_from_group(const char *, gid_t *);
+#ifndef __APPLE__
 const char *
 group_from_gid(gid_t, int);
+#endif
 __END_DECLS
 
 #endif

include/bsd/pwd.h

@@ -44,8 +44,10 @@
 __BEGIN_DECLS
 int
 uid_from_user(const char *, uid_t *);
+#ifndef __APPLE__
 const char *
 user_from_uid(uid_t, int);
+#endif
 __END_DECLS
 
 #endif

include/bsd/stdlib.h

@@ -51,6 +51,7 @@
 #include <stdint.h>
 
 __BEGIN_DECLS
+#if !defined(__APPLE__)
 #if !defined(__GLIBC__) || \
     !__GLIBC_PREREQ(2, 36) || \
     !defined(_DEFAULT_SOURCE)
@@ -60,6 +61,7 @@ uint32_t arc4random_uniform(uint32_t upper_bound);
 #endif
 void arc4random_stir(void);
 void arc4random_addrandom(unsigned char *dat, int datlen);
+#endif
 
 int dehumanize_number(const char *str, int64_t *size);
 

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) || \