build: add -j6 to our build scripts' "make" invocations.

build-linux

@@ -7,5 +7,5 @@ set -ex
 ./bootstrap
 ./configure
 make clean
-make
+make -j6
 

build-mingw

@@ -71,5 +71,5 @@ fi
 ./configure $HOSTFLAGS $BUILDFLAGS \
             CPPFLAGS="$CPPFLAGS" LDFLAGS="$LDFLAGS" \
             GTK_LIBS="$GTK_LIBS" LOL_LIBS="$LOL_LIBS"
-make
+make -j6