add possibility of using system googletest
conditional checks for dependencies
fixed non-matched if/endif expressions
Merge pull request #310 from sfabbro/master

conditional dependency check depending on build request

Here are more fixes/features to the build system:

    possibility to use system googletest using cmake FindGtest.cmake (which should be available with most cmake installations)
    will check for required dependencies only if user requests the features requiring those dependencies (client, libcvmfs, and server)
    a small fix for c-ares enabled curl check