rely on the system compiler (even if that means we need to be explicitly
conservative when it comes to optimizations). [1]
Simplify the creation of the multilib-related sub-directory tree on
powerpc64 and avoid leaving an empty directory behind on the way. [2]
Sync pkg-descr with lang/gcc7-devel, in particular after r442530 there.
PR: 235975 [1], 235964 [2], 231804 [2]
Submitted by: Piotr Kubaj <pkubaj@anongoth.pl> [1]
Discussed with: Piotr Kubaj <pkubaj@anongoth.pl> [2]
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