Automatically find homebrew packages if present.

[agatti]

This simple change sort of works around the need of playing with environment variables to let CMake pick up a more recent version of Bison on OSX if one is available. Currently not tested on anything but OSX+Homebrew, you may want to see if it doesn't create problems on either Linux or Windows, though.

[massix]

It seems like a good patch to me, and it works fine on an OSX environment. My guess is that this is harmless on a non-OSX environment as it will just fallback to the builtin find_package macro.

The only question I have is, are we sure that we aren't breaking the way environment variables are read? This patch is pushing things on top of 4 CMake variables that are frequently set passing the -D flag to cmake: what happens if I want to use a bison version that I have installed outside the brew's path?

e.g.: 1) $ cmake -DCMAKE_INCLUDE_PATH=/some/path .. at this point, the CMAKE_INCLUDE_PATH will be set to /some/path; 2) Once the find_package_prefer_brew macro will be called, will the highest priority be held by the environment or by the macro? Because to me, the macro is overriding the environment, as the CMAKE_INCLUDE_PATH will then read /usr/local/include;/some/path.

[agatti]

So, how about making the homebrew variables overriding automatic if and only if CMAKE_{MODULE,PROGRAM,INCLUDE,LIBRARY}_PATH are empty then? Maybe this can be restricted even further by applying an explicit check for OSX (well, darwin) and use plain find_package for Bison and Flex on anything that's not OSX.

[agatti]

I've made some minor changes to the script, hope these help solving the issues that have just been raised.

[esseks]

That's great. I have no possibility to test your code on Mac OS, but I will do some test builds on Windows and Linux to make sure nothing is broken. I agree with @massix in that we should not alter the environment in a way that could override user preferences, however I see you have fixed the bad bits.

In order to keep the build script clean, you should move your code to a separate file in cmake/ subdir and include it in the main script. You should also close your blocks with endif() and the like (without repeating the condition), to decrease verbosity.

[agatti]

There it is, hope it looks ok now. I didn't set a custom module path otherwise it would mess with values that might have been set by the user, so a full path to the homebrew support code was more or less in order (I'm not a CMake expert, after all).

[esseks]

Unfortunately I am quite busy right now, so it will take a bit before I can validate and finally merge the changes. In the meantime, if anyone is willing to give this changeset a try, I will welcome his/her feedback.