configure: Avoid warnings if av_always_inline is disabled

This macro is disabled if --enable-small or --disable-optimizations are set.
Currently, this leads to warnings about functions being defined but not used,
for functions in header files.

By defining av_always_inline as inline or as av_unused, we avoid these
warnings.

This doesn't make a normal build with --enable-small any larger,
since the compiler probably chooses not to inline these functions even
if they're marked as inline.

Originally committed as revision 26032 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -3246,7 +3246,11 @@ test -n "$malloc_prefix" &&
 
 if enabled small || disabled optimizations; then
     echo "#undef  av_always_inline"  >> $TMPH
-    echo "#define av_always_inline"  >> $TMPH
+    if enabled small; then
+        echo "#define av_always_inline inline"  >> $TMPH
+    else
+        echo "#define av_always_inline av_unused"  >> $TMPH
+    fi
 fi
 
 if enabled yasm; then