Add pkg-config file for COGL

Add pkg-config file for COGL

COGL should ship its own pkg-config file, obviously still pointing
to Clutter's compiler flags and linking options, for COGL-specific
variables that might be queried at configure time.

For instance, it's easier (and less verbose) to do:

  PKG_CHECK_EXISTS([cogl-gl-1.0],
                   [has_gl_backend=yes],
                   [has_gl_backend=no])

Than doing:

  AC_MSG_CHECKING([for GL support in COGL])
  cogl_backend=`$PKG_CONFIG --variable=cogl clutter-0.9`
  if test x$cogl_backend = xgl; then
    has_gl_backend=yes
    AC_MSG_RESULT([found])
  else
    has_gl_backend=no
    AC_MSG_RESULT([not found])
  fi