Build Fontconfig 2.11 in-tree on Linux

After applying this patch, on Linux you will need to

    ../configure
    make clean-rust-azure clean-fontconfig clean-rust-fontconfig

Fixes #1752.
Install expat-devel on Linux builders
Install gperf on Linux builders
Work around missing expat.pc on some RedHat-based systems
auto merge of #1780 : kmcallister/servo/linux-fontconfig, r=pcwalton

After applying this patch, on Linux you will need to

    ../configure
    make clean-rust-azure clean-fontconfig clean-rust-fontconfig

Fixes #1752.

Expect Fontconfig warnings and possibly worse breakage, but for me this makes it possible to use parallel layout at all.