ffmpeg: Add option to build without GPL.

[techdragon]

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• [] Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• [] Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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Small change, it should help when using homebrew to build shared libraries as part of CI toolchains where GPL licensed FFMPEG shared libraries are unsuitable. With this change the existing behaviour of GPL builds should remain the default and continue to work unchanged. Its possible the build flags may need more sophistication regarding without-gpl and conflicting flags/options to do with GPL licensed components of FFMPEG, but exhaustively installing and uninstalling and reinstalling the combinations of options and flags to be sure is not something I really want to run by hand. (which is why I haven't done a local install with this option)

Edit: checklist syntax

[DomT4]

args << "--enable-gpl" if build.with? "gpl"

Is the preferred syntax.

[techdragon]

Thanks, I was watching the Jenkins log output and saw the style check complaint. I've issued an update to the file.

[techdragon]

Fix mistake, I don't normally write ruby.

[ilovezfs]

==> brew style ffmpeg
==> FAILED
== /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/ffmpeg.rb ==
C:122:  1: Trailing whitespace detected.

1 file inspected, 1 offense detected

[ilovezfs]

Licensed shouldn't be capitalized.

It's FFmpeg not FFMPEG.

[techdragon]

Looks like it passes the CI test now. I checked the console output and noticed the brew audit in Jenkins isn't run with the --strict flag and Jenkins also only runs brew install not brew install --build-from-source. Will I need to brew install --build-from-source <formula> and brew audit --strict <formula> before this can be merged?

[ilovezfs]

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