build: Make COGL pass distcheck

build: Make COGL pass distcheck

Some changes to make COGL pass distcheck with Automake 1.11 and
anal-retentiveness turned up to 11.

The "major" change is the flattening of the winsys/ part of COGL,
which is built directly inside libclutter-cogl.la instead of an
intermediate libclutter-cogl-winsys.la object.

Ideally, the whole COGL should be flattened out using a
quasi-non-recursive Automake layout; unfortunately, the driver/
sub-section ships with identical targets and Automake cannot
distinguish GL and GLES objects.