The most complex problems were from dealing with switches reverted in
the meantime (gcc5, gmp6, ncurses6).
It's likely that darwin is (still) broken nontrivially.
Package changes from 3.6:
- CMake exports patch no longer necessary
- Cosmetic purity patch fix
- Build libc++ with private libc++abi headers visible from sources
- Work around bugs in lldb's configure scripts