ntl: add gmp option.

Closes Homebrew/homebrew#38065.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/ntl.rb

@@ -5,6 +5,8 @@ class Ntl < Formula
   url "http://www.shoup.net/ntl/ntl-6.2.1.tar.gz"
   sha1 "3b9ab3bedb0b2e9b5ee322d60745be5caf1c743f"
 
+  depends_on "gmp" => :optional
+
   bottle do
     cellar :any
     sha1 "b55331990e2df310d694d5ae67dc3a55d6f18fd1" => :mavericks
@@ -13,8 +15,10 @@ class Ntl < Formula
   end
 
   def install
+    args = ["PREFIX=#{prefix}"]
+    args << "NTL_GMP_LIP=on" if build.with? "gmp"
     cd "src" do
-      system "./configure", "PREFIX=#{prefix}"
+      system "./configure", *args
       system "make"
       system "make", "check"
       system "make", "install"