Refactor ./installdeps and add support for mingw-w64 cross builds on
Arch Linux. The latter does not work currently because of a bug in
libuuid in the mingw-w64 crt, and the wxmsw AUR package does not build.
This will hopefully be resolved soon.
Put a copy of FindOpenAL.cmake into our own CMakeScripts/ because
searching for include files with PATH_SUFFIXES of include/ does not work
for cross builds, as it is done in the system version.
53fb6b4371