mapnik: fix missing proj and libxml2 support

This commit provides three minor improvements:

1. Fix missing proj support. Fixes #91889.
2. Fix missing libxml2 support.
3. Improve build time with parallel build.

Fix missing proj support:
-------------------------
The 2019-07-09 update to proj version 6.1.1 in nixpkgs
commit d55cba68ff3 caused two problems for mapnik:

1. The separation of the dev output was not reflected in the mapnik
   expression. Since proj support is optional in the mapnik build,
   the build succeeded but no longer included proj support.

   See build log for: https://hydra.nixos.org/build/96274475.

2. Even if the mapnik expression was updated to use the separate
   dev output, it would still have dropped proj support because mapnik
   has not been updated for the proj 6 API.

This commit fixes the PROJ_INCLUDES to use the correct dev output and
also makes the PROJ_LIBS output explicit. In addition, it defines
ACCEPT_USE_OF_DEPRECATED_PROJ_API_H=1 since mapnik still does not have
a tagged release that supports the proj 6 api.

Looking to the future, it appears that the mapnik master branch includes
proj 6 support, so some checks were included to help insure that this
workaround gets removed once it is no longer required.

Fix missing libxml2 support:
----------------------------
Configure with "XMLPARSER=libxml2" to get libxml2 support. This has been
needed since mapnik 3.0.2. Remove XML2_INCLUDES and XML2_LIBS which
trigger a bug in the supplied SConstruct script (when used with
"XMLPARSER=libxml2.

In addition, libxml2 needs to be a propagatedBuildInput to avoid
errors building apacheHttpdPackages.mod_tile, python-mapnik, and
possibly other packages.

Enable parallel build:
----------------------
Setting enableParallelBuild does not result in a parallel build
with mapnik. However, setting JOBS=$(NIX_BUILD_CORES) in buildFlags
works and significantly reduces the build time.

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