giscanner: Add Optional Options for Codegen

This adds options to scannermain so that we can decorate functions with
macros as needed, so that we can use compiler annotations for symbol export
for example.

Options are also added to include headers before or after the main include
block at the top so that we can include headers as necessary in the
generated sources and/or headers, so that we could for example grab
definitions from those headers as needed, such as to grab definitions of
macros used for symbol export.

The testcodegen.py script has been updated as well to make use of this
functionality, if needed.

https://bugzilla.gnome.org/show_bug.cgi?id=732669
tests/Makefile.am: Update Generation of everything.[c|h]

Make use of the new options in codegen.py/testcodegen.py so that the
symbols in the generated everything.[c|h] can be annotated for export, and
the necessary headers to facilitate the compile-time symbol exporting can
be included appropriately.

https://bugzilla.gnome.org/show_bug.cgi?id=732669
MSVC Builds: Update Generation of everything.[c|h]

Like the update to tests/Makefile.am, update gi-tests-msvc.mak so that
everything.[c|h] can be generated with the symbols annotated for export and
the headers that define those annotations can be pulled in appropriately.

https://bugzilla.gnome.org/show_bug.cgi?id=732669