BUG #13988. Added ability to use system zlib.

On certain systems, especially when using system-qt/system-python, we'd rather
use system ZLIB as well, especially for building components that don't use
mangled ZLIB. This addresses that issue.

Change-Id: I8190b3d570fe0f2bf9c9b74a13cd3411cdaa6a29

Projects/zlib.cmake

-
 # zlib supports cmake. the only problem is that we need to remove the zconf.h
 # file.
-add_external_project(
+add_external_project_or_use_system(
   zlib
   # remove the zconf.h as a patch step.
   PATCH_COMMAND ${CMAKE_COMMAND} -E remove -f <SOURCE_DIR>/zconf.h

Projects/zlib.use.system.cmake

+find_package(ZLIB)
+
+add_extra_cmake_args(
+   -DZLIB_INCLUDE_DIR:PATH=${ZLIB_INCLUDE_DIR}
+   -DZLIB_LIBRARY:FILEPATH=${ZLIB_LIBRARY}
+)