Allow parallel make when the -j option is passed to build.sh.

Based on a patch to the UQM Mega-Mod by Ala-lala.

sc2/build/unix/build_functions

@@ -22,11 +22,11 @@ usage() {
 	echo "Main build script"
 	echo
 	echo "Syntax:"
-	echo "    ./build.sh <target>"
+	echo "    ./build.sh [-j[#JOBS]] <target>"
 	echo "    ./build.sh <target> config"
 	echo "    ./build.sh <target> depend"
 	echo "    ./build.sh <target> clean"
-	echo "    ./build.sh <target> install"
+	echo "    ./build.sh [-j[#JOBS]] <target> install"
 	echo "    ./build.sh cleanall"
 	echo "    ./build.sh distclean"
 	echo
@@ -103,6 +103,8 @@ build_depend() {
 
 # Compile the lot.
 # With the depend info set up, we can leave everything to make.
+# $1 - additional arguments to pass to make (at the moment just
+#      an optional -j arg for parallel builds).
 build_compile() {
 	local CFLAGS LDFLAGS TARGET_FILE DEPEND_FILE OBJDIR
 
@@ -119,7 +121,7 @@ build_compile() {
 			BUILD_ROOT= \
 			TARGET_FILE=$TARGET_FILE DEPEND_FILE=$DEPEND_FILE \
 			SED=$SED \
-			$MAKE -f Makefile.build "$TARGET_FILE"
+			$MAKE $1 -f Makefile.build "$TARGET_FILE"
 
 	eval "${TARGET}_post_build"
 }
@@ -170,12 +172,14 @@ build_check_dependencies() {
 }
 
 # Description: check if the program is compiled, and otherwise compile
+# $1 - additional arguments to pass to make (at the moment just
+#      an optional -j arg for parallel builds).
 build_check_compile() {
 	local NAME
 	eval NAME="\${${BUILD_PROJECT}_NAME}"
 	[ ! -e "$NAME" ] || return
 
-	build_compile || exit $?
+	build_compile "$1" || exit $?
 }
 
 # Make a directory path, with mode and owner specified.