in public headers, don't assume pre-C99 compilers have __inline keyword

include/features.h

@@ -24,6 +24,8 @@
 
 #if __STDC_VERSION__ >= 199901L || defined(__cplusplus)
 #define __inline inline
+#elif !defined(__GNUC__)
+#define __inline
 #endif
 
 #if __STDC_VERSION__ >= 201112L