Add the `m32` parameter to ./installdeps to fetch 32 bit multilib
dependencies on Fedora, Arch and Solus linuxes and add a cmake toolchain
file to build with them.
For MSYS2 support both an `m32` and an `m64` to choose deps for 32 bit
or 64 bit mingw targets.
Adjust build instructions from ./installdeps to support both usages.
Other changes:
- use --nogpgcheck --best --allowerasing for dnf invocations for Fedora,
the --nogpgcheck is necessary when Rawhide is confused about which
keys it has, and the latter two are needed for upgrades that remove
unneeded deps etc
- when finding GTK2, first try using pkg-config, and only fall back to
the cmake script when that fails, pkg-config works better for cross
builds
- add cairo to dep lists in ./installdeps for the time being
- for arch, list individual packages from base-devel instead of
base-devel that do not require gcc-libs because of potential conflicts
between gcc and gcc-multilib
Rafael Kitover