Build system: check for inconsistent settings (#10157)

`configure` currently detects when the docbook and hscolour tools aren't
available, and instead of failing outright (as it does for missing alex
and happy), sets some variables in mk/config.mk to tell `make` not to
build the documentation.

Sometimes, however, you want to really make sure all documentation gets
built, fully colourized. For example when making a release. To do so,
you can override the mentioned variables from mk/config.mk in
mk/build.mk (e.g. set HSCOLOUR_SRCS=YES).

This patch adds some error checking to make sure that doing so will not
result in weird build failures when those tools are still missing.

Test Plan: ran `make` a couple of times, with different mk/config.mk settings.

Reviewed by: austin

Differential Revision: https://phabricator.haskell.org/D1232

0 files changed, 0 insertions, 0 deletions