Revert "build: Remove multiple inclusion guards from config.mak."

This reverts commit 89b5829d.

Conflicts:

	configure

configure

@@ -3319,6 +3319,8 @@ config_files="$TMPH config.mak"
 
 cat > config.mak <<EOF
 # Automatically generated by configure - do not modify!
+ifndef FFMPEG_CONFIG_MAK
+FFMPEG_CONFIG_MAK=1
 FFMPEG_CONFIGURATION=$FFMPEG_CONFIGURATION
 prefix=$prefix
 LIBDIR=\$(DESTDIR)$libdir
@@ -3461,6 +3463,7 @@ SEEK_TESTS=$(print_enabled   -n _test $SEEK_TESTS)
 EOF
 
 echo "#endif /* FFMPEG_CONFIG_H */" >> $TMPH
+echo "endif # FFMPEG_CONFIG_MAK" >> config.mak
 
 # Do not overwrite an unchanged config.h to avoid superfluous rebuilds.
 cp_if_changed $TMPH config.h