Work around OSes with broken pkg-config files

For some reason Solaris patches gio-2.0.pc to remove the full path to binaries specified in it (like gdbus-codegen). This is broken: there's no reason to do this, and part of the point of putting these in the pkgconfig file in the first place is to give the absolute path of the binary that corresponds to that package.

But sure, let's fall back to AC_PATH_PROG in that case, and while we're at it, we might as well further fall back to AC_PATH_PROG if the pkgconfig file is missing the variable entirely.

Closes #66