FFmpeg iOS build script

This is a shell script to build FFmpeg libraries for iOS apps.

Tested with:

FFmpeg http://git.videolan.org/?p=ffmpeg.git;a=commit;h=f29cdbe1b59a0d997733b507041e15ec68cef00c
Xcode 5.0.2
https://github.com/libav/gas-preprocessor (support arm64)

Usage

To build everything:
```
  ./build-ffmpeg.sh
```
To build arm64 libraries:
```
  ./build-ffmpeg.sh arm64
```
To build fat libraries for armv7 and x86_64 (64-bit simulator):
```
  ./build-ffmpeg.sh armv7 x86_64
```
To build fat libraries from separately built thin libraries:
```
  ./build-ffmpeg.sh lipo
```

Download

https://downloads.sourceforge.net/project/ffmpeg-ios/ffmpeg-ios.tar.bz2

Currently, arm64 architecture is not included in the download.

Influences

https://github.com/bbcallen/ijkplayer/blob/fc70895c64cbbd20f32f1d81d2d48609ed13f597/ios/tools/do-compile-ffmpeg.sh#L7