ngspice: fix test and X11 dependency.

Formula/ngspice.rb

@@ -5,24 +5,25 @@ class Ngspice < Formula
   url 'http://downloads.sourceforge.net/project/ngspice/ng-spice-rework/24/ngspice-24.tar.gz'
   sha1 'a6bb0e65c1c07d48f1d3e4df44fc17f6262952eb'
 
+  option "with-x", "Build with X support"
   option "without-xspice", "Build without x-spice extensions"
 
-  depends_on :x11
+  depends_on :x11 if MacOS::X11.installed? or build.include? "with-x"
 
   def install
     args = %W[
       --disable-dependency-tracking
       --prefix=#{prefix}
       --with-editline=yes
-      --enable-x
     ]
+    args << "--enable-x" if build.include? "with-x"
     args << "--enable-xspice" unless build.include? "without-xspice"
 
     system "./configure", *args
     system "make install"
   end
 
-  def caveats;
-    "Note: ngspice is an X11 application."
- end
+  def test
+    system "#{bin}/ngspice", "-v"
+  end
 end