Fix Building the OpenCOLLADAValidator when CMAKE_DEBUG_POSTFIX is set.

When CMAKE_DEBUG_POSTFIX is set then building OpenCOLLADAValidator did only work for release builds. For Debug builds the linker still searched for the libraries without the defined postfix and failed.

I also have renamed the vars LIBXML2_LIBRARIES and PCRE_LIBRARIES to singular:
LIBXML2_LIBRARY and PCRE_LIBRARY for better readability.

If at any time in the future these vars will contain more than one lib then it is time to change that back to plural.