changed cmake to use a new build type for drd Tokutek/ft-engine#48

This lets us use CMAKE_BUILD_TYPE=drd instead of hijacking
RelWithDebInfo.

Also, make RelWithDebInfo synonymous with Release so we can build
MySQL/MariaDB that way.
Merge pull request #34 from Tokutek/drd-flags

changed cmake to use a new build type for drd Tokutek/ft-engine#48