Add @GIT2R_SRC_REGEX@ substitution to Makevars.in

* Use the substitution when configuring for building 'git2r' on
  Solaris.

* misc/build_Makevars.r: Add functionality to create the substitution
  when generating src/Makevars.in

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add 'tools/README.md'

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