configure.ac: don't require eglmesaext.h

On a build for a i.MX 6 compile fails with:

| .../cogl-egl-defines.h:38:10: fatal error: EGL/eglmesaext.h: No such file or directory
| 38 | #include <EGL/eglmesaext.h>

A egl.pc is provided, so in configure the tests for the existence of EGL/eglmesaext.h are not run and the error is reported during compile.

If one does not include EGL/eglmesaext.h in COGL_EGL_INCLUDES then the build succeeds.

Change to include EGL/eglmesaext.h only if it actually does exist.