enable C99-external_inline for icl

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/attributes.h

@@ -41,8 +41,12 @@
 #endif
 
 #ifndef av_extern_inline
+#if defined(__ICL) && __ICL >= 1210 || defined(__GNUC_STDC_INLINE__)
+#    define av_extern_inline extern inline
+#else
 #    define av_extern_inline inline
 #endif
+#endif
 
 #ifndef av_noreturn
 #if AV_GCC_VERSION_AT_LEAST(2,5)