package/libgit2: bump version to 1.4.2

Licence changes mostly concern vendored dependencies (that buildroot does
not use) and a new bundled PRNG used to reimplement mkstemp() - added
CC0 license for that.

Some buildsystem options also changed:
- THREADSAFE is now USE_THREADS
- BUILD_CLAR is now BUILD_TESTS, which the cmake infra already give.

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>