COMP: BuildTexFiles target should depend on SoftwareGuideExamples

Otherwise, without the (now deprecated, see policy CMP0050) SOURCE
parameter in add_custom_command, the build system will try to
build BuildTexFiles without first generating the .tex files generated
in the custom command specifying the ParseCxxExamples.py script.

Change-Id: Icc653eba2e8cffb019eb12e2dc62356fad84fc33