CMakeLists.txt: do not install .gitignore files

Prevents two .gitignore files from being installed:
  * Fixed the list(REMOVE_ITEM...) macro not actually removing the
    .gitignore file from the list. The list contains full paths to
    files, thus we need the full path to .gitignore as well, which
    requires a file(GLOB...);
  * Fixed install(DIRECTORY...) to add a PATTERN match removing the
    .gitignore file.

Signed-off-by: Timothée Ravier <tim@siosm.fr>