Homebrew build fails when Gambit is installed

[chiefnoah]

See this thread for context.

When you have a separate Gambit Scheme install on your system, the homebrew build for MacOS may fail if it does not use the libgambit built as part of the Gerbil build.

According to @drewc, this means we're likely missing a -L flag somewhere in the build.

[vyzo]

this should be fixed on master