Use relative path to set cmake module path

This has actually no effect on a "normal" build, but allows to use
libgit2 as a part of a larger project via CMake's ADD_SUBDIRECTORY()

Closes #2087
Cleanup FindIconv.cmake

Doesn't change anything. Just removes stuff that was probably missed
to remove when this was imported.
Find and use a MacPorts version of libiconv. Fixes #2017.

- Add correct -I, -L and -l flags
- Search for libiconv in /opt/local/[include|lib] before in the
  system path. See #2017 for details.
- Give splitted -L and -l arguments to pkg-config