Build poppler from git

Many issues in Evince are really bugs in Poppler, so make it easier for people to build Poppler from JHBuild, hopefully this will attract more contributors or at least facilitate things for existant ones.

For licensing issues, Poppler consists of three repos, one for encoding files (to properly render CJK and Cyrillic), one to host the pdf files used for testing, and lastly the source repo.