Perform a substantial cleanup of the build system with respect to
building and installing testcases.

First, Makefile.decl has been renamed glib.mk and substantially
expanded.  We intend to add more stuff here in the future, like canned
rules for mkenums, marshallers, resources, etc.

By default, tests are no longer compiled as part of 'make'.  They will
be built when 'make check' is run.  The old behaviour can be obtained
with --enable-always-build-tests.

--disable-modular-tests is gone (because tests are no longer built by
default).  There is no longer any way to cause 'make check' to be a
no-op, but that's not very useful anyway.

A new glibtests.m4 file is introduced.  Along with glib.mk, this
provides for consistent handling of --enable-installed-tests and
--enable-always-build-tests (mentioned above).

Port our various test-installing Makefiles to the new framework.

This patch substantially improves the situation in the toplevel tests/
directory.  T...