plan9port: use C compiler from Nix

The install script was escaping the Nix environment on Mac OS by using
`xcrun -sdk macos clang` as its C compiler.  Using the Nix compiler
required declaring the necessary frameworks as inputs and patching
build scripts to assume MacOS 10.12 (and not try to detect).

So cached derivations prior to this would probably not work on all
intended target machines.

This *might* also fix installCheck on Darwin on Hydra.

Other minor fixes:

* Disable parallel building due to a race with a missing y.tab.h
* Use NIX_CFLAGS_COMPILE/NIX_LDFLAGS instead of trying to synthesize
  something like them.
* X11 dependencies aren't used on Darwin when the windowing system
  is correctly detected

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