giflib: disable default X11 sniffing

Giflib automatically sniffs for X11 on the system during compile, and can throw
linking errors if it detects its availability but can't link to it.

Closes Homebrew/legacy-homebrew#42050.

Closes Homebrew/legacy-homebrew#42921.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Formula/giflib.rb

@@ -1,8 +1,6 @@
 class Giflib < Formula
   desc "GIF library using patented LZW algorithm"
   homepage "http://giflib.sourceforge.net/"
-  # 4.2.0 has breaking API changes; don't update until
-  # things in $(brew uses giflib) are compatible
   url "https://downloads.sourceforge.net/project/giflib/giflib-4.x/giflib-4.2.3.tar.bz2"
   sha256 "0ac8d56726f77c8bc9648c93bbb4d6185d32b15ba7bdb702415990f96f3cb766"
 
@@ -15,9 +13,27 @@ class Giflib < Formula
 
   option :universal
 
+  depends_on :x11 => :optional
+
   def install
     ENV.universal_binary if build.universal?
-    system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking"
+
+    args = %W[
+      --prefix=#{prefix}
+      --disable-dependency-tracking
+    ]
+
+    if build.without? "x11"
+      args << "--disable-x11" << "--without-x"
+    else
+      args << "--with-x" << "--enable-x11"
+    end
+
+    system "./configure", *args
     system "make", "install"
   end
+
+  test do
+    assert_match /Size: 1x1/, shell_output("#{bin}/gifinfo #{test_fixtures("test.gif")}")
+  end
 end