On win32 the tests are built but they are not run automatically by the
build system.
A minimal 'check' target (depending on the tests being built) can
simply run them and log to the console their success/failure.
The win32 build system repeatedly defines some basic variables
(notably program names and flags) and C sources compilation rules.
They can be factored out to a common Makefile, to be included in every
other Makefile.win32.
Makefile.am and Makefile.win32 should not duplicate content, as this
leads to breaking the build when they are not kept in sync.
This can be avoided by listing sources, headers and common build
variables/rules in a Makefile.sources file.
In order to further simplify the test makefiles, the utility functions
are now in a static library, which gets linked to all the tests and
benchmarks.
