Include libxml2 and zlib as required libraries

libxml2 is a required library, but we only find out that when the build
fails to link against it, if it is not present. The same for zlib.

We use find_library to find these two libraries and print nice fail
messages if they are not found.
543952eb87Wander Lairson Costa committed on 3/20/2018, 6:15:02 PMparente966d37
1 file changedLine totals unavailable