Add CMake Policy settings.

I set version 2.6 as the baseline since cmake_minimum_required does so.
CMP0037 denies target names reserved by generators, e.g. test.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Check whether CMP0037 is available or not.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #228 from omasanori/cmp0037

Add CMake Policy settings.
WIP: Add (picrin socket) and (srfi 106).

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>