Coccinelle for PostgreSQL development [2/N]: autoconf support

This second patch adds support for coccicheck to configure.ac and related
files (in particular Makefile.global.in). At this point, I have
deliberately not added support for pgxs so extensions cannot use coccicheck
through the PostgreSQL installation.

The semantic patches are expected to live in cocci/ directory under the
root and the patch uses the pattern cocci/**/*.cocci to find all semantic
patches. Right now there are no subdirectories for the semantic patches,
but this might be something we want to add.

The coccicheck target is used in the same way as for the Linux kernel, that
is, to generate a patch with all changes suggested by the semantic patches,
you type:

make coccicheck MODE=patch

Linux has support for a few more variables: V to set the verbosity, J to
use multiple jobs for processing the semantic patches, M to select a
different directory to apply the semantic patches to, and COCCI to use a
single specific semantic patch rather than all available. I have not added
support for this right now, but if you think this is valuable, it should be
straightforward to add.
--
Best wishes,
Mats Kindahl, Timescale