Bash lets you create environment variables using this format:
There is a Readme.html file included in the xerces tarball that has some build instructions for Unix-style systems. It says that the XERCESCROOT environment variable needs to be set to the path to the Xerces-C source code directory. In other words, if you untar'ed the source to /home/Jorek/software/xerces-c, then you need to export XERCESCROOT with this:
That last directory is created when you untar the source tarball, and the numbers will depend on which version of the software you downloaded.
To read the Readme.html file that came in the tarball, open a web browser, select File->Open File, and browse to the directory that has it. It has lots of good stuff in it.