Fix bug: building on MACOSX

1) Update the CmakeLists.txt to fix missing symbols in linking phase as:
```
"_iconv", referenced from:
_avcodec_decode_subtitle2 in libavcodec.a(utils.o)
"_iconv_close", referenced from:
_avcodec_open2 in libavcodec.a(utils.o)
_avcodec_decode_subtitle2 in libavcodec.a(utils.o)
"_iconv_open", referenced from:
_avcodec_open2 in libavcodec.a(utils.o)
_avcodec_decode_subtitle2 in libavcodec.a(utils.o)
```

2) The ffmpeg lib should be rebuild in order to avoid missing symbols as:
```
"___sincos_stret", referenced from:
_ff_mdct_init in libavcodec.a(mdct_float.o)
_ff_ps_init in libavcodec.a(aacps.o)
"___sincosf_stret", referenced from:
_ff_ps_init in libavcodec.a(aacps.o)
```

3) Here is the new build commands that works fine for me, and should work fine too for everybody on all macosx:
In the ppsspp root path, type
```
cd ffmpeg
./mac_x86-64.sh
cd ..
mkdir build-mac
cd build-mac
cmake ../
make
```

PS1: you should have homebrew, which is installed by:
```
ruby -e "$(curl -fsSL https://raw.github.com/Homebrew/homebrew/go/install)"
```
PS2: if you missing command "yasm" when building ffmpeg, then try install it by:
```
brew install yasm
```