fixup fontconfig.rb (linux mint 17, gcc 4.9)

After brew updates, some of the issues in #164 are solved,
the only remaining issue is the docs.

This commit disables compiling the docs, and instead tries
to install the pregenerated documentation that comes with
the tarball. Without a patch like this, my compilation stops
with errors when trying to generate the docs.

See compilation notes here:
http://www.linuxfromscratch.org/blfs/view/svn/general/fontconfig.html
fixup blockers for cairo.rb

I had two issues

(1) it explicitly relies on XQuartz. fix with `unless os.Mac?`
(2) the cairo test suite fails, this apparently is caused by
gcc 4.9, see here:

https://bugs.archlinux.org/task/40313

I fix it by adding an extra cflag to the make invocation as
described in the report.