Mac/iOS: Include default brew-installed FFMPEG directory path

[kubark42]

This reduces build-environment complexity by including default paths for FFPEG as installed by brew. Tested to work both for macOS and iOS builds.

[kubark42]

P.S. Seems that the CI is failing on the archiving step

wget http://sourceforge.net/projects/sshpass/files/sshpass/1.05/sshpass-1.05.tar.gz/download -O sshpass.tar.gz
--2017-09-26 22:27:53--  http://sourceforge.net/projects/sshpass/files/sshpass/1.05/sshpass-1.05.tar.gz/download
Resolving sourceforge.net... 52.84.125.208, 52.84.125.204, 52.84.125.163, ...
Connecting to sourceforge.net|52.84.125.208|:80... connected.
HTTP request sent, awaiting response... 302 Moved Temporarily
Location: https://sourceforge.net/#!/projects/sshpass/files/sshpass/1.05/sshpass-1.05.tar.gz/download [following]
--2017-09-26 22:27:53--  https://sourceforge.net/
Connecting to sourceforge.net|52.84.125.208|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 652 [text/html]
Saving to: ‘sshpass.tar.gz’
sshpass.tar.gz      100%[===================>]     652  --.-KB/s    in 0s      
2017-09-26 22:27:53 (88.8 MB/s) - ‘sshpass.tar.gz’ saved [652/652]
tar zxf sshpass.tar.gz
tar: Unrecognized archive format
tar: Error exit delayed from previous errors.
shell_session_update
/Users/travis/.travis/job_stages: line 57: shell_session_update: command not found

[wang-bin]

it's for macOS only, not iOS

[kubark42]

it's for macOS only, not iOS

@wang-bin I'm unclear if you're referring to the travis CI error, or the PR. FWIW, mac includes both macOS and iOS, whereas macx is exclusively macOS[1]. This code builds both iOS and macOS successfully, although I haven't tried to build and run an iOS app using the outputted QtAV binaries. If there is an easy way to test that, I'm happy to give it a shot.

[1] https://stackoverflow.com/a/18520177