Update R&friends to use our X11 libs (#2053)

* Update R&friends to use our X11 libs

Add `depends_on()`'s for R, Cairo, and Tk so that they use the Spack X
bits.

* Conditionalize X stuff on variant

Another step towards hooking up the X stuff.

There's still discussion happening on the issue, but this is better
than the 'everything must build with X' state that it was in.

var/spack/repos/builtin/packages/cairo/package.py

@@ -33,6 +33,12 @@ class Cairo(AutotoolsPackage):
 
     version('1.14.0', 'fc3a5edeba703f906f2241b394f0cced')
 
+    variant('X', default=True, description="Build with X11 support")
+
+    depends_on('libx11', when='+X')
+    depends_on('libxext', when='+X')
+    depends_on('libxrender', when='+X')
+    depends_on('libxcb', when='+X')
     depends_on("libpng")
     depends_on("glib")
     depends_on("pixman")