Further fixes to $DESTDIR handling #1512

Job	Run time
Ubuntu GCC, default flags with AVX2, Code Coverage (x3)	14m 9s
FreeBSD Clang (x0.5)	44s
macOS GCC with BLAS (x1)	31m 29s
Ubuntu GCC (assert, x1)	17m 21s
Cygwin GCC (x0.5)	28m 34s
MinGW GCC (x0.5)	19m 4s
MSVC (x1, excl test)	16m 30s
Nemo.jl	2m 39s
Ubuntu Clang (x1)	19m 28s
Ubuntu GCC via CMake (no check)	10m 45s
	2h 40m 43s

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