At the same time, change the library sonames for -3.0 to just -3.
This is necessary since the 2.99 releases installed libraries like
libgtk-3.0.so.0.9903.0, and we want to prevent the library version
number from jumping back. So 3.0 will have libgtk-3.so.0.0.0.
This commit does a number of things:
- remove some dead wchar configury from configure.ac and gdkconfig.h
- repurpose gdkconfig.h as header that contains GDK_WINDOWING_foo
macros for each included backend, include it in gdk.h and install
it in $includedir instead of below $libdir
- drop the backend from the library names
- build libgdk-3.0.la as a convenience lib and include it in libgtk-3.0.la
It does not yet enable building multiple backends at the same time.
