cogl/cogl-gtype-private.h: Fix Build on non-GCC

Commit 1b2dd815 (Registers gtypes for all public objects and structs)
introduced GCCism's in its use of varargs, which broke the build of Cogl
on other non-GCC compilers, such as Visual Studio.

Define the COGL_GTYPE_DEFINE_BASE_CLASS and COGL_GTYPE_DEFINE_CLASS macros
using ISO-style varargs so that the build of Cogl can be fixed on non-GCC
compilers.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
framebuffer: Fix linkage from C++ applications

Prevent Cogl function names from being mangled when a C++ compiler is
being used by adding some missing COGL_{BEGIN,END}_DECLS guards.

https://bugzilla.gnome.org/show_bug.cgi?id=728628

Reviewed-by: Neil Roberts <neil@linux.intel.com>