- Up to now, the libdir was hardcoded to "lib" inside pugixml.pc and
the install directory of pugixml.pc was "lib/pkgconfig"
- Adds support for lib and lib64 by using CMAKE_INSTALL_LIBDIR variable
There's really never a reason to *not* want this installed. If an option
is needed to specify installing in a versioned subdirectory, this option
should be explicitly described rather than hidden in something else.
As an added bonus, this makes the CMake install code slightly *less*
complicated.
