Allow out of tree builds

[orbea]

This allows for out of tree builds and creates all of the objects and the 7zip static archive in the objs/ directory. Unfortunately libdeflate writes to the source directory instead of the build directory, but it does still work without that. To fix libdeflate that should be handled in their repo first assuming that they are receptive.

Example:

mkdir -p /tmp/build
cd /tmp/build
make -f /path/to/maxcso/Makefile

[orbea]

Travis CI for osx never fails to be painful...

[orbea]

The duplication of rules is unfortunate

Agreed. I think I got rid of that now.

Reference: https://stackoverflow.com/questions/11441084/makefile-with-multiples-rules-sharing-same-recipe