Build system backports for 1.6

[haampie]

• Allow system versions of libwhich and libblastrampoline (Allow system versions of libwhich and libblastrampoline #43000)
• Add missing USE_SYSTEM_LIBWHICH initialization (Add missing USE_SYSTEM_LIBWHICH initialization #43014)

Previously included, now dropped:

• Symlink 7z when USE_SYSTEM_P7ZIP (Symlink 7z when USE_SYSTEM_P7ZIP #43005)

[staticfloat]

Agreed, these all should be backported. Thanks @haampie!

[vchuravy]

@haampie can you rebase onto backports-release-1.6?

[KristofferC]

@nalimilan had some issue with this I think (the p7zip symlink IIRC)?

[nalimilan]

Ah yes, thanks for pinging me. I've commented at #43005 (comment). I don't see why that PR should be backported to 1.6. If we do it for this kind of feature, we could do the same for many PRs. What's more, I don't think it's correct in its current implementation.

[haampie]

I see @nalimilan's point as I only considered install prefixes other than the system root in 43005, so the 7z commit is dropped.

[haampie]

@KristofferC this PR only backported USE_SYSTEM_LIBWHICH in the end and dropped the p7zip changes. Can that still be added or is that too late now?