Do not force brewisms, rather make USE_HOMEBREW an explicit configure option

[Sergey Fedorov]

It is wrong to force a specific package manager, even if it happens to be installed. There are several package managers on macOS, and while it is up to developers which to support if any, an assumption that macOS == Homebrew is incorrect and may lead to breakages. Specifically, find_program(HOMEBREW_EXECUTABLE brew) should not be done unconditionally but rather sit inside a macro.

[Sameer Agarwal]

This sound reasonable, we would be happy to accept a patch fixing this following instructions at

http://ceres-solver.org/contributing.html

[Sergiu Deitsch]

I suggest we remove the corresponding dependency to Homebrew on macOS targets. We do not use similar logic for other package managers either. Setting CMAKE_PREFIX_PATH should be done by the user or the toolchain.

@alexsmac Any objections?

[Sergey Fedorov]

Thank you for addressing this!