cogl-1.0: cope with missing x11 headers

cogl-1.0: cope with missing x11 headers

Fix commit 3e96efc4dd87 ("cogl-1.0: : don't require eglmesaext.h") if
x11 headers are missing.

In that case AC_CHECK_HEADERS will report the egl headers missing, as
by default on unix it builds for X11 as the platform but the compiler
will fail to find the X11 headers.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Tested-by: Andrey Zhizhikin <andrey.z@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>