Use pkg-config more extensively in build
We now make use of pkg-config on Linux, BSD, and macOS systems to find the headers and libs for the following dependencies, where we weren't previously:
- enet
- fmt (on macOS only)
- gloox's dependencies on macOS:
- gnutls
- gmp
- nettle
- libcurl
- libogg
- libsodium
- miniupnpc (on macOS only)
- openAL
- openGL
- vorbis
- zlib
Please see revision (phab:D3611) for details.
NOTE: All those building on macOS will need to rebuild their libraries (build-osx-libs.sh --force-rebuild)
Tested by: Langbart (macOS), Freagarach (Ubuntu 18.04), Jenkins CI (Debian Buster)
Comments by: Stan, wraitii, Freagarach
Differential Revision: https://code.wildfiregames.com/D3611