Replace the custom Makefile by automake. In addition drop the hacky
libwidgetextra-uninstalled.pc and configure the library like every other
optional library, but check if the library is not disabled.
the list of dependent libraries for static library linkage, so they go in libs.private. This was causing the duplication of CFLAGS into the final link (which was clumsy but largely harmless), but also build failures for people with funky multi-part options in their CFLAGS (which is where it was found), because some of the flags didn't survive pkg-config's manipulations and came out corrupted.