CMake build does not obey specified INKSCAPE_DATADIR and INKSCAPE_LIBDIR

[Roman Hargrave]

If INKSCAPE_DATADIR or INKSCAPE_LIBDIR are specified when invoking CMake (e.g. by -DINKSCAPE_DATADIR=...), their values are not reflected in the generated config.h, rather the default values ("${CMAKE_INSTALL_PREFIX}/usr/share", "${CMAKE_INSTALL_PREFIX}/usr/lib") are used.

A fix would be to go about setting these values in the same manner as PACKAGE_LOCAL_DIR - e.g. check for a preexisting definition before defining it.

[Qantas94Heavy (qantas94heavy)]

Hi, would you like to submit this as a merge request on GitLab? It'll be quicker and easier for us to merge in. Here's the repo: https://gitlab.com/inkscape/inkscape

If not, we can still merge in your patch for you, so if you could let us know that'd be great. Thank you!