Add support to override pkg-config with meson.override_find_program #16647

Job	Run time
Arch / PyPy	20m 9s
Arch Linux	20m 7s
CUDA (on Arch)	7m 27s
Fedora	19m 57s
Ubuntu Rolling (gcc, g++)	19m 28s
Gentoo	21m 25s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 24s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 19s
OpenSUSE	16m 37s
Ubuntu Rolling (clang, clang++)	21m 55s
Ubuntu Bionic	14m 41s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	16m 3s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 41s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 26s
	3h 52m 39s

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