Create a CMake macro to handle project specific options

`cmake_dependent_option` just doesn't work like I always think it should so we should create our own called `serac_project_specific_option` that encapsulates the behavior we want.

For example:

* `ENABLE_BENCHMARKS` is a global BLT created option, it is used for multiple projects. Allow a user to turn that on and honor it
* Unless the project specific one is set `SERAC_ENABLE_BENCHMARKS` then honor that one.
* Always create a `SERAC_ENABLE_BENCHMARKS` because the rest of our build system is guarded on that

Start by locating existing uses of cmake_dependent_option, ENABLE_BENCHMARKS, and SERAC_ENABLE_BENCHMARKS in the build configuration. Review how BLT options are created and consumed, then define completion as a project-specific macro that consistently applies the stated global and project-specific option behavior and always creates the SERAC option.

Written by the indexing model from the issue text.