Meson: Fix environment when cross building
Submitted by Xavier Claessens
When cross building with meson, the env must contain the host toolchain and the target tool chain must be written in a config file.
Currently cerbero always set the target toolchain in env so build.py has an ugly hack that unset CC and CXX variables when cross building.
A proper fix is probably to load both envs in config.py and switch env depending on the recipe being built.