The new target installs client API modules to EGL_DRIVER_INSTALL_DIR.
They are used by st/egl.

The client APIs are built from OpenGL and OpenVG state trackers.  For
this to work, st/vega is modified to produce a static library,
libvega.a, instead.  st/es is also not needed any more.  It is removed
and --with-state-trackers=es is replaced by --enable-gles-overlay.

As st/egl now has its own client API modules, this solves the ABI issue
between st/egl and client APIs, as long as the client API modules are
distributed with st/egl.  Plus, this allows st/egl to support OpenGL
with non-Gallium libGL.so.