Add OpenAL-specific define to fix `error: redefinition of 'struct timespec'` when compiling with MinGW.
`timespec` is already defined within MinGW, but probably detection does not work correctly.
Partially fixes#3372
The filter_audio API is not working for qTox/Tox and thus disabled by default via compiler switch. In current qTox UI, it is not configurable, even when this switch is enabled. According to issue #3194 the library is no longer maintained as well. I don't see any reason to keep it in qTox.
Fixed install_libfilteraudio.sh to work on windows
bootstrap.bat now invokes sh bootstrap.sh (for convenience)
Updated INSTALL.md to reflect simplified bootstrapping procedure
