build: Remove multiple inclusion guards from config.mak.

config.mak is no longer included multiple times; the guards are pointless.

configure

@@ -3248,8 +3248,6 @@ config_files="$TMPH config.mak"
 
 cat > config.mak <<EOF
 # Automatically generated by configure - do not modify!
-ifndef LIBAV_CONFIG_MAK
-LIBAV_CONFIG_MAK=1
 LIBAV_CONFIGURATION=$LIBAV_CONFIGURATION
 prefix=$prefix
 LIBDIR=\$(DESTDIR)$libdir
@@ -3391,7 +3389,6 @@ SEEK_TESTS=$(print_enabled   -n _test $SEEK_TESTS)
 EOF
 
 echo "#endif /* LIBAV_CONFIG_H */" >> $TMPH
-echo "endif # LIBAV_CONFIG_MAK" >> config.mak
 
 # Do not overwrite an unchanged config.h to avoid superfluous rebuilds.
 cp_if_changed $TMPH config.h