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.
The config.guess script tries to detect various ARM optimization flags
via /proc/cpuinfo. This is not only impure, but the detection is also
broken on multicore machines; on my quad-core system the machine type is
autodetected as 'neon neon neon neon-unknown-linux-gnueabihf':
checking build system type... Invalid configuration `neon': machine `neon' not recognized
configure: error: /nix/store/bafimhdj1yaxj6m1hvq7wvhwwizc939x-bootstrap-tools/bin/sh ./config.sub neon
neon
neon
neon-unknown-linux-gnueabihf failed
builder for ‘/nix/store/1npm2358bpvclj5w7fqjjwg72vbb0d79-gmp-6.0.0a.drv’ failed with exit code 1
Override the system type with the output of GNU config.guess to avoid
the autodetection.
