ctest: add CTest scripts

- Add CTestConfig.cmake script to use the Testing driver provided by
  CMake.
- Add the valgrind.supp to summarize valgrind outputs.

To run, use the following command:

  ctest -S CTestConfig.cmake -VV
cmake: best detection pkg-config dependencies

If the user does not use -DWITH_GTK, -DWITH_GTKSOURCEVIEWMM in the CMake
option, the CMakeLists accept to prepare Makefile without correct
dependencies.

We need to detect available pkg-config dependencies.