The old logic for BUILD_SHARED_LIBS and BUILD_SHARED_AND_STATIC_LIBS
would produce two targets with the same name and different build
rules. The Makefile generator tolerated this, but the Ninja generator
raised an error. Fix the logic so that only one shared and one static
target gets built, and make the pugixml target an ALIAS of the one
dictated by BUILD_SHARED_LIBS.