XALANC-812: Replace use of !NDEBUG with XALAN_DEBUG

[rleigh-codelibre]

Add new macros:

• XALAN_DEBUG (to replace _DEBUG and !NDEBUG); defaults to ON for Debug builds, else OFF.
• XALAN_CRT_DEBUG (to replace the above combined with _MSC_VER for memory leak checking); defaults to ON for Debug builds when using MSVC, else OFF.

Both are cache variables with auto-detected defaults which may be overridden if needed.