class-loader: cmake error if gtest is not found

[bulwahn]

During configure of class-loader, it first warns:

| CMake Warning at /home/bulwahn/openembedded-core/build/tmp-eglibc/sysroots/i686-linux/usr/share/catkin/cmake/test/gtest.cmake:99 (message):
| gtest not found, C++ tests can not be built. You can run 'svn checkout
| http://googletest.googlecode.com/svn/tags/release-1.6.0 gtest' in the root
| of your workspace
| Call Stack (most recent call first):
| /home/bulwahn/openembedded-core/build/tmp-eglibc/sysroots/i686-linux/usr/share/catkin/cmake/all.cmake:131 (include)
| /home/bulwahn/openembedded-core/build/tmp-eglibc/sysroots/i686-linux/usr/share/catkin/cmake/catkinConfig.cmake:20 (include)
| CMakeLists.txt:8 (find_package)

and then fails with:

| -- skipping gtest 'class_loader_utest' in project 'class_loader'
| CMake Error at test/CMakeLists.txt:16 (target_link_libraries):
| Cannot specify link libraries for target "class_loader_utest" which is not
| built by this project.
|