Switch to non-recursive build process

* macros/mm-pkg.m4: New file, defining the MM_PKG_CONFIG_SUBST macro.
* macros/mm-python.m4: New file, defining the MM_CHECK_MODULE_PYTHON
macro to replace the old and messy AM_CHECK_PYTHON_HEADERS.
* acinclude.m4: Remove file which defined AM_CHECK_PYTHON_HEADERS.

* Makefile.am: Merge the contents of the Makefile.am files from all
subdirectories into a single top-level Makefile.am, with the exception
of docs/user-guide/Makefile.am.  Add the full subdirectory prefix to
all listed files and change the Automake variable names accordingly.
Get rid of the intermediate static libraries previously created for
each source subdirectory, and list the source files directly in the
toplevel target's SOURCES variable.  Overall, refactor and modernize
the build rules and organization.
* **/Makefile.am: Except for docs/user-guide/Makefile.am, remove the
Makefile.am files recursively from all subdirectories.

* config.h.in: Remove file, and use autoheader to generate it.
* autogen.sh: Replace with a modern minimalistic script which lets
autoreconf do the bulk of the work.
* configure.ac: Massive refactoring.  Adapt to the new non-recursive
build organization.  Modernize the M4 and shell code, and make use of
newer Autoconf and Automake constructs to simplify many checks.  Also
make minor corrections to a number of tests and definitions.

* glom/application.cc: Remove the need for GLOM_ICON_DIR by setting
the window icon by name instead of loading the image file directly.
Use PACKAGE_VERSION in place of VERSION.
* glom/dialog_existing_or_new.cc: Use a subdirectory of GLOM_DOCDIR
in place of GLOM_EXAMPLES_DIR.  Remove GLOM_EXAMPLES_DIR_ALTERNATIVE.

* glom/utils_ui.cc: Replace DATADIR by GLOM_DATADIR.
* glom/libglom/connectionpool.cc: Remove GLOM_SAVE_DATADIR hack, which
is not necessary anymore as DATADIR is not globally defined anymore.
* glom/libglom/connectionpool_backends/postgres_self.cc: ditto,
* glom/libglom/spawn_with_feedback.cc: ditto,
* glom/main.cc: ditto. Also replace LOCALEDIR by GLOM_LOCALEDIR, and
output PACKAGE_STRING instead of VERSION in response to the --version
command-line option.  Replace PACKAGE and VERSION by their modern
Automake equivalents.
* glom/xsl_utils.cc: Remove GLOM_SAVE_DATADIR hack, and replace
GLOM_XSLTDIR with a subdirectory of GLOM_PKGDATADIR.

* glom/mode_data/box_data_calendar_related.cc: Replace GLOM_GLADEDIR
by a combination of GLOM_PKGDATADIR and a subdirectory name.
* glom/mode_design/print_layouts/print_layout_toolbar_button.cc: Use
a subdirectory of GLOM_PKGDATADIR in place of GLOM_ICON_DIR.
* glom/utility_widgets/layouttoolbar.cc: ditto,
* glom/utility_widgets/layouttoolbarbutton.cc: ditto.

* glom/python_embed/glom_python.cc: Include generated config.h for the
definition of GLOM_ABI_VERSION_UNDERLINED.
* glom/python_embed/python_module/py_glom_module.cc: ditto.