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.
diff --git a/include/dirent.h b/include/dirent.h
index 726067f..d445f80 100644
--- a/include/dirent.h
+++ b/include/dirent.h
@@ -9,7 +9,7 @@
 
 #define __NEED_ino_t
 #define __NEED_off_t
-#ifdef _BSD_SOURCE
+#if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)
 #define __NEED_size_t
 #endif
 
@@ -53,16 +53,13 @@
 #define DT_WHT 14
 #define IFTODT(x) ((x)>>12 & 017)
 #define DTTOIF(x) ((x)<<12)
+int getdents(int, struct dirent *, size_t);
 #endif
 
 #ifdef _GNU_SOURCE
 int versionsort(const struct dirent **, const struct dirent **);
 #endif
 
-#ifdef _BSD_SOURCE
-int getdents(int, struct dirent *, size_t);
-#endif
-
 #if defined(_LARGEFILE64_SOURCE) || defined(_GNU_SOURCE)
 #define dirent64 dirent
 #define readdir64 readdir
@@ -72,10 +69,8 @@
 #define versionsort64 versionsort
 #define off64_t off_t
 #define ino64_t ino_t
-#ifdef _BSD_SOURCE
 #define getdents64 getdents
 #endif
-#endif
 
 #ifdef __cplusplus
 }