Fix gitignore

Current gitignore state is inconsistent: files with mask `*.cmake` is ignored
but there is a file FindICU.cmake.
Remove temporary CMake files from gitignore (use _build directory)
Remove unnecessary 'add_dependencies' commands

If 'target_link_libraries' used, no need to add dependency explicitly.

From documentation:

  If a library name matches that of another target in the project a dependency
  will automatically be added in the build system to make sure the library
  being linked is up-to-date before the target links.
Merge pull request #370 from ruslo/remove.dependencies

Remove unnecessary 'add_dependencies' commands
Merge pull request #363 from ruslo/gitignore.fix

Fix gitignore
Removed comments in http/src/CMakeLists.txt.