Proposed enhancement: Use CMakeDependentOption to simplify option logic

Is there a reason you're not using the [`CMakeDependentOption`](https://cmake.org/cmake/help/v3.0/module/CMakeDependentOption.html) CMake module for handling option dependencies?

i.e., `ENABLE_GMOCK` depends on `ENABLE_GTEST`, `ENABLE_FRUIT` depends on `ENABLE_FORTRAN` etc.

It would be easy to work up a PR to include this capability. CMake support goes as far back as at least 3.0 if not earlier.

Start by locating the CMake option definitions for ENABLE_GMOCK, ENABLE_GTEST, ENABLE_FRUIT, and ENABLE_FORTRAN. Read CMake's CMakeDependentOption documentation and compare the current dependency behavior. Done means the dependent options are handled consistently without breaking existing configuration behavior.

Written by the indexing model from the issue text.