It should now be easier to read, and more featureful. Among other
tweaks, we now print a summary of dependencies and build options, and
the documentation is generated with more sensible breadcrumbs and
builds properly with Sphinx 1.3.

REVIEW: 126075

ecm_generate_headers() now allows/forces the caller to collect the paths
of the generated headers, so that they can be passed to the install
command.  This avoids issues of unexpected files being in the CamelCase
includes directory, both from previous builds and because of
case-insensitive file systems.

MODULE_NAME is removed, as it is no longer desirable or necessary.
Instead, the headers are placed directly in the output directory
(usually CMAKE_CURRENT_BUILD_DIR).

Overall, this makes ecm_generate_headers() behave much more like other
file generation macros (like the Qt ones).

The old syntax is still supported for now, to make the porting effort
easier.

REVIEW: 115765