6.4.4: Build improvements

Fixes some puzzling CMake build errors. Turns out the problem was, sometimes there are still autoconf-generated config headers in the source tree, and the CMake build would prefer those over the ones which CMake itself had generated in the binary tree. Bad things would happen if the two contained incompatible configuration.

Also, uses pkg-config if the pg_config script is not available. Expect pkg-config to become the default over time.