Support MAKEOPTS in addition to MAKE_OPTS (#89)

bin/ruby-build

@@ -128,9 +128,13 @@ build_package() {
 
 build_package_standard() {
   local package_name="$1"
-  if [ -z "${MAKE_OPTS+defined}" ]; then
+
+  if [ -z "${MAKEOPTS+defined}" ]; then
+    MAKE_OPTS="$MAKEOPTS"
+  elif [ -z "${MAKE_OPTS+defined}" ]; then
     MAKE_OPTS="-j 2"
   fi
+
   { ./configure --prefix="$PREFIX_PATH" $CONFIGURE_OPTS
     make $MAKE_OPTS
     make install