configure: separate simple EGL clients from others

configure: separate simple EGL clients from others

Separate simple EGL clients from other simple clients. This allows to
build either simple-shm or simple-egl, whichever you want. We avoid
linking libEGL and GLESv2 into simple-shm, and we can build simple-shm
even if nothing provides EGL, GLESv2, or wayland-egl APIs.

Change the options in configure --help from --enable to --disable, since
these are enabled by default, and you would normally only ever give the
--disable flavor. Add descriptions.

Remove the #define BUILD_SIMPLE_CLIENTS since it is not used.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>