New way to generate the XMP library (Using the CMake OBJECT LIBRARY concept)

[piponazo]

In this Pull Request I decided to improve the way in which we are handling the XMP support. These are the things I modified:

• I moved the XMPSRC variable from the main CMakeLists.txt file to xmpsdk/CMakeLists.txt. Now the variable is only available in the scope of the xmpsdk folder.
• Usage of the CMake OBJECT library concept to reuse object files. When EXIV2_ENABLE_XMP=ON and EXIV2_ENABLE_LIBXMP=ON, those object files are used to generate the xmp static library. When EXIV2_ENABLE_XMP=ON and EXIV2_ENABLE_LIBXMP=OFF, those object files are appended to the list of sources for the exiv2lib library.
• Remove the dependency on ${EXPAT_LIBRARIES} from many targets. That dependency is already inherited from the exiv2lib target.
• Continue with the unification of lower_case CMake commands.