Provide a way to disable Serac benchmarks

[chapman39]

@tupek2 is adding Smith benchmarks. Smith needs a way to disable Serac benchmarks while running Smith's.

Serac benchmarks should not be run within Smith, since because Smith doesn't update Serac very often, so they won't be as accurate.

Eventually we should look into better cmake options #1282, but for now cmake_dependent_option should be re-added, imo.

[tupek2]

This seems reasonable to me. There is a clear pattern. Ideally, all our options can be like this, so I don't even have to specify project specific ones (DENABLE_CODEVELOP, for example doesnt exist, so we have to do it per project).

[tupek2]

Thinking about this a bit... I think people get confused because
ENABLE_SERAC_BENCHMARKS=ON does not turn on ENABLE_BENCHMARKS.
ENABLE_BENCHMARKS is like the "house master value" in this design. It must be turned on for the local values to turn on/off.
We can either flip the logic, so ENABLE_BENCHMARKS just changes the local defaults, which can then always be turned on/off independently. Or we can keep the current design, but if we do, it might make it clearer if we change the local names to:
DISABLE_SERAC_BENCHMARKS=ON (all these disables would default to false) to limit the confusion potential a bit.

Then its clearer that ENABLE_BENCHMARKS turns them on for all projects, and per project we can disable them if needed.

[chapman39]

DENABLE_CODEVELOP, for example doesnt exist, so we have to do it per project

this variable doesn't exist, because it's not an option in BLT. here is a list of all blt options https://github.com/LLNL/blt/blob/develop/cmake/BLTOptions.cmake