Protect config.mak against multiple inclusion

Some of the values set here are modified by the main makefiles
and should not be reset when config.mak is included again.

Originally committed as revision 19417 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2479,6 +2479,9 @@ echo "#define FFMPEG_CONFIG_H" >> $TMPH
 echo "#define FFMPEG_CONFIGURATION \"$(c_escape $FFMPEG_CONFIGURATION)\"" >> $TMPH
 echo "#define FFMPEG_DATADIR \"$(eval c_escape $datadir)\"" >> $TMPH
 
+echo "ifndef FFMPEG_CONFIG_MAK" >> config.mak
+echo "FFMPEG_CONFIG_MAK=1" >> config.mak
+
 echo "FFMPEG_CONFIGURATION=$FFMPEG_CONFIGURATION" >> config.mak
 echo "prefix=$prefix" >> config.mak
 echo "LIBDIR=\$(DESTDIR)$libdir" >> config.mak
@@ -2609,6 +2612,7 @@ enabled asmalign_pot &&
 echo "#define EXTERN_PREFIX \"${extern_prefix}\"" >> $TMPH
 
 echo "#endif /* FFMPEG_CONFIG_H */" >> $TMPH
+echo "endif # FFMPEG_CONFIG_MAK" >> config.mak
 
 # Do not overwrite an unchanged config.h to avoid superfluous rebuilds.
 cmp -s $TMPH config.h &&