[3.0] contrib: update CMake calls to match 4.0

So we can cherry pick contrib updates easier in the future (needed by !6102).

Also includes:

• building (outdated) cmake in snap
• remove redundant mkdir in aom script to match 4.0

And from 4.0:

• split build and install phases
• use local variables instead of passing all configure flags
• disable pthread in Win32 vncclient
• update glew from 1.7.0 to 2.1.0 so it can be built with CMake
• use jobserver to build CMake targets
• remove unneeded DENABLE_WIN32_IO=OFF from libebml
• contrib: remove the CMakeCache.txt at every run
• update openjpeg to 2.5.0
• contrib: mimic the Android CMake toolchain
• contrib: don't set the build mode in the toolchain file