Warn if git executable can not be found and default version to 'unknown'

This will avoid confusion by generating package that look like the release
one.

Before:

  -- CTKResEdit version: 0.1.0

After:

  CMake Warning at CMake/ctkVersionSource.cmake:34 (message):
  GIT_EXECUTABLE not found.  CTKAppLauncher_VERSION_SOURCE default to
  'unknown'.
Call Stack (most recent call first):
  CMakeLists.txt:58 (include)

  -- CTKResEdit version: 0.1.0-unknown