This makes sure the internal versions of these libraries bundled with
the GZDoom source code is used. This prevents the system from building
GZDoom for dynamic linking with incompatible external libraries (see
<https://forum.zdoom.org/viewtopic.php?f=2&t=64633>).
# Conflicts:
# glslang/glslang/CMakeLists.txt
# glslang/spirv/CMakeLists.txt