Enable conditional use of -Werror in compile

:Release Notes:
sleepd can now be compiled with all warnings treated as errors by
specifying -DWEBOS_USE_WERROR=TRUE on the cmake command line.

:Detailed Notes:
It is useful for a developer to be able to compile with -Werror to
ensure all warnings are either suppressed or dealt with. But enabling
-Werror generally can completely break a build when a new version of
the compiler adds new warnings.

This commit adds a command line option to the cmake script to turn on
-Werror for a standalone bulld of sleepd, without impacting a full
image build.

It also fixes some warnings found this way, and adds generation of
Doxygen documentaiton fo the source code.

:Testing Performed:
- desktop build

:QA Notes:

:Issues Addressed:

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>

Change-Id: I844e563f9f299e1926cc54ce09c10bd69fcee7a4
Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-on: https://g2g.palm.com/3857
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Build Verification