make it possible to compile with -std=gnu99/c99 and use extern inlines

When compiled with -std=gnu99/c99 __GNUC_GNU_INLINE__ is not defined

Signed-off-by: Peter S. Mazinger <ps.m@gmx.net>

include/features.h

@@ -433,7 +433,7 @@ uClibc was built without large file support enabled.
  */
 #if __GNUC_PREREQ (2, 7) && defined __OPTIMIZE__ \
     && !defined __OPTIMIZE_SIZE__ && !defined __NO_INLINE__ \
-    && (defined __extern_inline || defined __GNUC_GNU_INLINE__)
+    && (defined __extern_inline || defined __GNUC_GNU_INLINE__ || defined __GNUC_STDC_INLINE__)
 # define __USE_EXTERN_INLINES	1
 #endif