Use pkg-config in Autotools #2852

Job	Run time
GCC build regression check (no tests)	1m 32s
macOS-x86 Clang with BLAS (x2)	15m 14s
macOS-M1, Clang (x3)	9m 6s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 1s
MinGW GCC (x0.5)	11m 1s
MSVC (x1)	9m 16s
Nemo.jl (temporary branch)	6m 21s
Ubuntu Clang with examples (x5)	12m 42s
Ubuntu GCC, Code Coverage (x10)	30m 41s
Ubuntu GCC with NTL (assert, x2)	8m 32s
	1h 58m 26s

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