[akonadi] Add switch to build development/testing tools optionally
Review Request #126850 - Created Jan. 22, 2016 and submitted
BUILD_TESTING=TRUE currently alters the install image, this change introduces BUILD_TOOLS to conditionally build
BUILD_TESTING=TRUE however will still trigger BUILD_TOOLS=TRUE.
Before the merge into akonadi.git, these testing tools subdirs were already (partially) moved into the root CMakeLists.txt, albeit without a switch, but that got lost with the merge again.
As a bonus, LibXML2 detection is moved into xml subdir where it is actually used. However, I've noticed akonadixml defines HAVE_LIBXML2 and it actually also builds without it - so I've made it optional at the same time. Requesting comments on that since I don't know if it was made required for a specific reason. ;)