build: Don't fail if libX11 isn't installed

build: Don't fail if libX11 isn't installed

configure.ac would previously refuse to complete if libX11 wasn't
installed, even if we'd disabled GLX and weren't building an X11 EGL
platform.  Make the check simply set the no_x variable that's used (but
never set) immediately below for what looks like this very case.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Dan Nicholson <dbn.lists@gmail.com>