do not define internal libc macros

macros like `__USE_XOPEN` are internal to libc and shouldn't be defined
by the user. the proper macro to define is `_XOPEN_SOURCE`.

defining `_XOPEN_SOURCE_EXTENDED` shouldn't be needed anymore since
according to the manpage (`man feature_test_macros`):

> defining _XOPEN_SOURCE with a value of 500 or greater produces the same
> effects as defining _XOPEN_SOURCE_EXTENDED.

but define it anyways, just in case.

also define _BSD_SOURCE and _DEFAULT_SOURCE

src/nnn.c

@@ -29,7 +29,15 @@
  */
 
 #define _GNU_SOURCE
+#define _BSD_SOURCE
+#define _DEFAULT_SOURCE
 #define _FILE_OFFSET_BITS 64 /* Support large files on 32-bit glibc */
+#define _XOPEN_SOURCE_EXTENDED
+#undef  _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700L
+#ifndef NCURSES_WIDECHAR
+#define NCURSES_WIDECHAR 1
+#endif
 
 #if defined(__linux__) || defined(MINGW) || defined(__MINGW32__) \
 	|| defined(__MINGW64__) || defined(__CYGWIN__)
@@ -57,19 +65,6 @@
 #endif
 #include <sys/wait.h>
 
-#ifdef __linux__ /* Fix failure due to mvaddnwstr() */
-#ifndef NCURSES_WIDECHAR
-#define NCURSES_WIDECHAR 1
-#endif
-#elif defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__) \
-	|| defined(__APPLE__) || defined(__sun)
-#ifndef _XOPEN_SOURCE_EXTENDED
-#define _XOPEN_SOURCE_EXTENDED
-#endif
-#endif
-#ifndef __USE_XOPEN /* Fix wcswidth() failure, ncursesw/curses.h includes whcar.h on Ubuntu 14.04 */
-#define __USE_XOPEN
-#endif
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -99,9 +94,6 @@
 #include <unistd.h>
 #include <stddef.h>
 #include <stdalign.h>
-#ifndef __USE_XOPEN_EXTENDED
-#define __USE_XOPEN_EXTENDED 1
-#endif
 #include <ftw.h>
 #include <pwd.h>
 #include <grp.h>