egl/main: remove undefined X11_LIBS automake variable

egl/main: remove undefined X11_LIBS automake variable

The EGL library has some references to x11 but it gets the link flags
from the XCB_DRI2_LIBS if and only if HAVE_EGL_PLATFORM_X11 is true.

The X11_LIBS variable was probably coming from a PKG_CHECK_MODULES (x11)
earlier in history.

If it is possible to have HAVE_EGL_DRIVER_GLX without HAVE_EGL_PLATFORM_X11
then the link flags for libX11 should be passed. However, it won't come
from X11_LIBS which is undefined.

Reported-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>