mingw: Bump docker build to Fedora 21.

The version of mingw in Fedora 21 works as well.
This is the first version to depend on libwinpthread.

Also add an autogen.sh invocation which caused the previous
build to fail after the 'make check' barrier was removed.

mingw/Dockerfile

-FROM fedora:20
+FROM fedora:21
 MAINTAINER opus@xiph.org
 
 # Linux build.
@@ -16,4 +16,4 @@ RUN git clone https://git.xiph.org/opusfile.git
 WORKDIR opusfile
 RUN git pull
 RUN make -C mingw
-RUN ./configure --host=i686-w64-mingw32 --prefix=${PWD}/mingw PKG_CONFIG_PATH=${PWD}/mingw/lib/pkgconfig && make && make check && make install
+RUN ./autogen.sh && ./configure --host=i686-w64-mingw32 --prefix=${PWD}/mingw PKG_CONFIG_PATH=${PWD}/mingw/lib/pkgconfig && make && make check && make install