get-gauche.sh shouldn't use make -j without options

Hi,

at least gnu make doesn't limit the parallelism if one uses -j without
options. => you get very high system load building gauche and the music
sometimes stops playing ;-)

Something like this helps here, but I didn't look into portability.

diff --git a/get-gauche.sh b/get-gauche.sh
index f46e5c9..445635e 100755
--- a/get-gauche.sh
+++ b/get-gauche.sh
@@ -418,7 +418,7 @@ function do_install {
             ./configure "--prefix=$prefix" $configure_args
             MAKE=make
             if hash gmake 2>/dev/null; then MAKE=gmake; fi
-            $MAKE -j
+            $MAKE -j $(nproc)
             if [ "$skip_tests" != yes ]; then
                $MAKE -s check
             fi