The library regression test now accepts 2176 objects for a successful test,
which is the number we currently have for mingw64 (down from the previous
value of 2251).

This adds a 'check' target to the top level Makefile which enables you
to run the usual regression tests in the scripts folder after a full
build. Works on all supported platforms.

Fix up the install/uninstall targets to properly handle the case that the build directory doesn't exist.

Add convenience targets to the toplevel Makefile which install additional abstractions and externals after a light build. E.g.: make light pdlua_ext

Fix an editing blunder which broke incremental builds, and add removal of the build dir to the clean target in the toplevel Makefile.