FindXercesC: Add support to `XercesC_DIR` for finding XercesC on non-system paths
Lots on internal modules support to XXX_DIR
or its equivalent.
But it's not available for the XercesC.
This path may be used for finding the local XercesCConfig.cmake
file, such as CURL done.
Tested on CMake 3.27.9 on Linux (Debian 9)
Edited by Alex Overchenko