Fix GNU/POSIX basename headers.

Including glibc's <libgen.h> will result in the user getting the POSIX
version of basename always, regardless of when it is included relative
to <string.h>. Prior to this patch, our implementation would result in
the one that's included first winning.

Bug: http://b/25459151
Change-Id: Id4aaf1670dad317d6bbc05763a84ee87596e8e59
diff --git a/libc/include/string.h b/libc/include/string.h
index 2c50cfa..32d4a18 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -115,18 +115,18 @@
 extern int    strcoll_l(const char *, const char *, locale_t) __purefunc;
 extern size_t strxfrm_l(char* __restrict, const char* __restrict, size_t, locale_t);
 
-#if defined(__USE_GNU) && !defined(__bionic_using_posix_basename)
+#if defined(__USE_GNU) && !defined(basename)
 /*
  * glibc has a basename in <string.h> that's different to the POSIX one in <libgen.h>.
  * It doesn't modify its argument, and in C++ it's const-correct.
  */
+
 #if defined(__cplusplus)
 extern "C++" char* basename(char*) __RENAME(__gnu_basename) __nonnull((1));
 extern "C++" const char* basename(const char*) __RENAME(__gnu_basename) __nonnull((1));
 #else
 extern char* basename(const char*) __RENAME(__gnu_basename) __nonnull((1));
 #endif
-#define __bionic_using_gnu_basename
 #endif
 
 extern void* __memchr_chk(const void*, int, size_t, size_t);