mandoc: support cross as far as possible
mandoc: support cross as far as possible

mandoc can't be cross compiled easily since all configure checks rely on
executing a binary built using $CC. We can however still do “native”
cross compilation like pkgsStatic and pkgsLLVM. To facilitate this, the
following changes are required:

* Set the correct AR to use in configure.local

* Inform users that cross compilation will fail via a throw if the build
  platform can't execute the host platform's binaries.

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