You can now specify `BUILD_COLOUR_NAME_MAP=OFF` to disable building the
gperf-based colour map, which also removes the requirement for gperf at
build time.
Rather than allowing the build to continue without gperf, we should
fail with an error so that the colour behaviour does not change in a
backward incompatible manner. The old colour behaviour should continue
to work going forward.
This resolves #1868.
Older versions of gperf use the `register` keyword which is deprecated
in C++17, and warnings are treated as errors.
This disables the warning on gperf's output (colour-names.hh). I also
renamed `colour-names.cc` -> `colour-names.hh` because we're including
it as a header, not compiling it separately.
This resolves #1865.
