contains(QT_CONFIG, egl) and CONFIG += egl is the only sane way to test
for and pull in EGL headers and libs. This is particularly important when
trying to be robust and guard against half-broken sysroots on embedded
where a naive PKGCONFIG += egl breaks.

Also add an EGL_WAYLAND_BUFFER_WL define to keep wayland-egl compiling.
We are not testing for that in any config tests may cause a failure in sysroots
that have parts of Mesa thrown in but pick ip an older EGL header from the
vendor's driver.

Change-Id: I7b7e6a7a91e78dbda5b6954ad08761298c538efc
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>