Make cogl-sdl.h a top-level header

This makes it so that cogl-sdl.h is a top-level header no longer
automatically included by cogl.h. This avoids lots of warnings building
the conformance tests and examples due to SDL.h warning when
__STRICT_ANSI__ isn't defined.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit f7536985437dc85c26b33d1bbe1b7f3d4b32476a)
Build and install cogl-path as a shared library

Since we now have more time to ensure that Clutter is updated to check
for the now separate cogl-path package as part of its build
configuration we are now making the package split, in line with Cogl
master.

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