feature test macros: make _GNU_SOURCE enable everything

previously, a few BSD features were enabled only by _BSD_SOURCE, not
by _GNU_SOURCE. since _BSD_SOURCE is default in the absence of other
feature test macros, this made adding _GNU_SOURCE to a project not a
purely additive feature test macro; it actually caused some features
to be suppressed.

most of the changes made by this patch actually bring musl in closer
alignment with the glibc behavior for _GNU_SOURCE. the only exceptions
are the added visibility of functions like strlcpy which were BSD-only
due to being disliked/rejected by glibc maintainers. here, I feel the
consistency of having _GNU_SOURCE mean "everything", and especially
the property of it being purely additive, are more valuable than
hiding functions which glibc does not have.

include/sys/stat.h

@@ -88,11 +88,8 @@ int mkfifoat(int, const char *, mode_t);
 int futimens(int, const struct timespec [2]);
 int utimensat(int, const char *, const struct timespec [2], int);
 
-#ifdef _BSD_SOURCE
-int lchmod(const char *, mode_t);
-#endif
-
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+int lchmod(const char *, mode_t);
 #define S_IREAD S_IRUSR
 #define S_IWRITE S_IWUSR
 #define S_IEXEC S_IXUSR