Fix TypeError when building from source with gcc compiler in some MacOS version

[niu541412]

In some deprecated MacOS, e.g.10.13 High Sierra, when compiling formulae with --cc=gcc-XX, homebrew will raise

TypeError: Return value: Expected type Symbol, got type String with value "gcc-XX".

Convert "other" string to symbol to avoid this.

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[carlocab]

Thanks!

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @niu541412!