ppsspp 0.9.9.1: new formula

[PPSSPP](http://ppsspp.org) is an open source emulator of Sony PlayStation Portable (PSP).

Here are some notes worth mentioning:

1. Their source archive couldn't be built without bringing in some dependencies from git repositories. That's why it's directly tapping git tag which spends quite a long time to clone large submodules including its own `ffmpeg`.

2. Even though `ffmpeg` submodule is fully cloned, it's not compiled anyways, but only pre-built libraries are seemingly used on linkage. A patch would be needed if we really want to build everything from source.

3. The build system looks for a version with `git distribute` which is not available on staging step. As a workaround, its version is manually set to the one of formula being installed.

4. `-DCMAKE_BUILD_TYPE=None` had to be removed from `std_cmake_args`, otherwise it won't build. The build system assumes `Release` mode when no build type is specified.

5. Final product is an `.app` package. A symlink is provided for running on command line.