Fix RelWithDebInfo name bugs, and use full optimization.

Gromacs was previously using a different name (RelWithDebugInfo) in
a few places, which caused the build types not to match, and
flags did not propagate correctly. Second, we now add our own
optimization flags at the end of the flag variables, to make it more
likely they override the CMake default choices. Otherwise, the -O2
flag at the end of the default RelWithDebInfo flags might revert our flags.

Change-Id: I4b7e220fbb2d625416e939e71d44412601394c0f