Add option GIT2R_USE_ICONV to Makevars.in

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Move files from 'tools' to 'misc'

* Prepare to use the 'tools' directory to ship auxiliary files for
  the configuration.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add configuration scripts

* Add scripts for configuring/building git2r.
  - tools/config.guess
  - tools/config.sub
  - tools/install-sh

* From
  - git clone http://git.savannah.gnu.org/cgit/automake.git
  - git checkout 24bf687ed03f1259dea1f0b6588c9e5c1e57366c

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Check for darwin and iconv in configure.ac

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Fix compiler warnings in git2r C code

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Merge pull request #60 from ropensci/configure-iconv

Configure iconv