The library can be used also on Darwin using it like this:

  NIX_REDIRECTS='foo=bar' \
  DYLD_INSERT_LIBRARIES=${libredirect}/lib/libredirect.so \
  DYLD_FORCE_FLAT_NAMESPACE=1 \
  some_program

So let's actually not hardcade gcc and add Darwin to meta.platforms.

No other changes seem to be required.

Signed-off-by: aszlig <aszlig@nix.build>

This is to make sure we get the correct shared library suffix of the
target platform. While for example on Darwin it would even work with the
hardcoded .so prefix it's IMHO a bit nicer to have the actual native
extension.

Signed-off-by: aszlig <aszlig@nix.build>

This is just a sanity check on whether the library correctly wraps the
syscalls and it's using the "true" executable for posix_spawn() and
execv().

The installCheckPhase is not executed if we are cross-compiling, so this
shouldn't break cross-compilation.

One thing I'm not actually sure is whether ${coreutils}/bin/true is
universally available on all the platforms, nor whether all the
functions we use in the test are available, but we can still fix that
after we've found out about that.

Signed-off-by: aszlig <aszlig@nix.build>

libredirect: Add support for Darwin