meson: build libEGL

meson: build libEGL

This is based heavily on Daniel Stone's work for the same, rebased on
master and with a number of TODO's fixed.

This does not implement glvnd (which is coming in a later patch)

Meson builds egl slightly differently than autotools, namely it doesn't
build an intermediate shared library. It doesn't do this because meson
doesn't have problems with the name of the library being dynamically
generated, so the glvnd and non-glvnd code can follow the same path.

v2: - Don't reuse variable (Eric E.)

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Tested-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>