Draft: meson cross-build with internal dependencies

I could not push in the MR !131 on @ao2's branch, so I created a new MR. The original message is:

Here are some changes to the meson files to make it easier to cross-build evince using MinGW and subprojects, see #1071

Compilation still does not succeed as there are some fixes needed in the subprojects, and evince still does not build when using gtk master (#1072)

I think all except the last commit (the one marked XXX) could be merged already, but I am marking the MR as WIP for now as I am still learning meson.

I rebase it, and made some other changes. The branch is untested.