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:

  -- CTKAppLauncher version: 0.1.12

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)


  -- CTKAppLauncher version: 0.1.12-unknown

Fixes #55
Merge branch '55-tweak-ctkVersionSource'

* 55-tweak-ctkVersionSource:
  Warn if git executable can not be found and default version to 'unknown'
Update driver script to ensure GIT_EXECUTABLE is propagated.

Fixes #55
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:

  -- CTKAppLauncher version: 0.1.12

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)

  -- CTKAppLauncher version: 0.1.12-unknown

See #55
Merge branch '55-tweak-ctkVersionSource'

* 55-tweak-ctkVersionSource:
  Warn if git executable can not be found and default version to 'unknown'
  Update driver script to ensure GIT_EXECUTABLE is propagated.