Moved cpputest CMake variables into CPPUTEST_ namespace (Again).

 * Fixed conditional for extension tests build.
Add support in CMake to build CppUTest as sub-project.

This allows projects to use CppUTest like this.

    add_subdirectory(cpputest)
    find_package(CppUTest REQUIRED HINTS ${CMAKE_BINARY_DIR} )

    include_directories("${CPPUTEST_INCLUDE_DIRS}")

    add_executable( test_all ... )
    target_link_libraries( test_all ${CPPUTEST_LIBRARIES} )