Make it easier to use self-built poppler

TL;DR with this change, Cmake now properly respects CMAKE_PREFIX_PATH

Details:

• make sure pkg-config finds poppler in non-standard path (https://cmake.org/cmake/help/v3.11/module/FindPkgConfig.html)
• now with that, something like mkdir build; cd build; cmake -DCMAKE_INSTALL_PREFIX=/build/tmp -DCMAKE_BUILD_TYPE=debug ../ works again.