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.
auto merge of #1780 : kmcallister/servo/linux-fontconfig, r=metajack

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.