Use C style comments

Update various locations which use C99 single line comments to conform to
C90 style comments. Found with: make CFLAGS="-std=C90"
Use g_snprintf instead of snprintf

Use g_snprintf for consistency with the rest of gobjectmodule.c
configure.ac: Use -std=c90 and error on declaration-after-statement

Replace gcc option of -std=c9x with c90 and add
-Werror=declaration-after-statement
This ensures we keep compatibility with msvc builds.
tests: Conditionalize usage of regress typelib in test_properties

Unconditional usage of regress breaks tests when PyGObject is built without
cairo.
tests: Conditionalize usage of GTK+ in tests_generictreemodel

This allows running make check without GTK+ installed.
tests: Move class definition depending on GTK+ within function evaluation

Move the definition of WindowWithSizeAllocOverride inside of the test
function call to so it is lazily defined. This avoids problems running tests
on systems without GTK+ installed.
Use ffi_call directly instead of g_callable_info_invoke

Cleanup internal callable cache and state tracking by removing multiple
counting schemes for differently sized "in" and "out" argument arrays.
Use a single count based on the total number of arguments passed to C
(inclusive of instance argument and GError exception where applicable).
Size all state tracking arrays to the same size and ensure argument cache
indices always line up with these arrays. This cleans up logic which was
required by g_callable_info_invoke for splitting "in" and "out" arguments
up.

Cleanup array marshaling which can now rely on the new scheme which ensures
the "arg_values" array always points to the correct location for length
argument values.

Cache the ffi_cif struct in PyGICallableCache via GIFunctionInvoker and
related GI methods. Overall, these changes can give a performance boost of
almost 2x for simple function calls (see ticket for micro benchmarks).

https://bugzilla.gnome.org/show_bug.cgi?id=723642
build: Update release-news to use srcdir

Use $(top_srcdir)/NEWS for pulling in news items to ensure
"make release-news" works in a vpath build environment.
release 3.11.91
configure.ac: post release version bump to 3.11.92