cmake: Disable in-source builds

In-source build are generally discouraged, see eg:
* http://www.cmake.org/Wiki/CMake_FAQ#I_run_an_out-of-source_build_but_CMake_generates_in-source_anyway._Why.3F
* http://www.cmake.org/Wiki/CMake_FAQ#What_is_an_.22out-of-source.22_build.3F

Moreover:
* Creating an in-source build makes it impossible to create a Debian
  package using this directory, because our package building mechanism
  needs to create a temporary out-of-source build tree
* Maintaining `.gitignore` file for in-source builds is a nuisance

For these reasons this commit removes entries for in-source builds from
`.gitignore` and makes it impossible to run such a build using `cmake`.