capnp: add --without-shared option.

Formula/capnp.rb

@@ -6,12 +6,19 @@ class Capnp < Formula
   sha1 '18ce1a404c2bf68e6625e44927bfe6b67186cb15'
 
   needs :cxx11
+  option 'without-shared', 'Disable building shared library variant'
 
   def install
-    system "./configure", "--disable-debug",
+    args = ["--disable-debug",
             "--disable-dependency-tracking",
             "--disable-silent-rules",
-                          "--prefix=#{prefix}"
+            "--prefix=#{prefix}"]
+
+    if build.without? "shared"
+      args << "--disable-shared"
+    end
+
+    system "./configure", *args
     system "make", "check"
     system "make", "install"
   end