Fix LevelDB builds for modern gcc versions

LevelDB fails to compile with the current compiler flags on a Debian box running gcc 4.7.1 due to an undefined 'ssize_t'. Other users are also seeing this failure: https://github.com/rvagg/node-levelup/issues/224.

The standalone LevelDB project builds with -std=c++0x which fixes the above error.
Merge pull request #135 from sharvil/patch-1

Fix LevelDB builds for modern gcc versions