Change `mruby-config` to print usage on invalid arguments.

Or no argument at all.

mrbgems/mruby-bin-config/mruby-config

 #!/bin/sh
 
+print_help()
+{
+  echo "Usage: mruby-config [switches]"
+  echo "  switches:"
+  echo "  --cflags                    print flags passed to compiler"
+  echo "  --cflags                    print flags passed to compiler"
+  echo "  --cflags                    print flags passed to compiler"
+  echo "  --ldflags                   print flags passed to linker"
+  echo "  --ldflags-before-libs       print flags passed to linker before linked libraries"
+  echo "  --libs                      print linked libraries"
+  echo "  --libmruby-path             print libmruby path"
+  echo "  --help                      print this help"
+  exit 0
+}
+
+if [ $# -eq 0 ]; then
+   print_help
+fi
+
 while [ $# -gt 0 ]; do
   case $1 in
     --cflags) echo MRUBY_CFLAGS;;
@@ -7,14 +26,7 @@ while [ $# -gt 0 ]; do
     --ldflags-before-libs) echo MRUBY_LDFLAGS_BEFORE_LIBS;;
     --libs) echo MRUBY_LIBS;;
     --libmruby-path) echo MRUBY_LIBMRUBY_PATH;;
-    --help) echo "Usage: mruby-config [switches]"
-            echo "  switches:"
-            echo "  --cflags                    print flags passed to compiler"
-            echo "  --ldflags                   print flags passed to linker"
-            echo "  --ldflags-before-libs       print flags passed to linker before linked libraries"
-            echo "  --libs                      print linked libraries"
-            echo "  --libmruby-path             print libmruby path"
-            exit 0;;
+    *) print_help;;
   esac
   shift
 done