meson: De-dup rpath arguments #16820

Job	Run time
Arch Linux	22m 14s
CUDA (on Arch)	7m 30s
Arch / PyPy	20m 36s
Fedora	20m 44s
Ubuntu Rolling (gcc, g++)	19m 49s
Gentoo	20m 36s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 56s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 21s
Ubuntu Rolling (clang, clang++)	22m 14s
OpenSUSE	16m 30s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 57s
Ubuntu Bionic	15m 7s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 6s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 33s
	3h 57m 13s

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