Add support to override pkg-config with meson.override_find_program #16744

Job	Run time
Arch Linux	20m 11s
Arch / PyPy	20m 16s
CUDA (on Arch)	7m 49s
Fedora	19m 44s
Ubuntu Rolling (gcc, g++)	19m 57s
Gentoo	22m 35s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 55s
OpenSUSE	16m 22s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 3s
Ubuntu Bionic	15m 15s
Ubuntu Rolling (clang, clang++)	21m 9s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 29s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 1s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 50s
	3h 54m 36s

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