ffmpeg-devel: fix compilation for OS X 10.7 through 10.12

[aeiouaeiouaeiouaeiouaeiouaeiou]

Description

PR #25416 is stuck in moderation, however after testing #26520 this patch should be added to ffmpeg-devel.
I'll rebase it if #26775 is merged first.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.6.8 10K549 x86_64
Xcode 4.2 4C199

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@mascguy for port ffmpeg-devel.

[aeiouaeiouaeiouaeiouaeiouaeiou]

@mascguy thanks!