configure.ac: refactor avcodec check

Use PKG_HAVE_WITH_MODULES to declare the automake conditional and do the
actual pkg-config lookup.

This will also change the output in case of failure, from:


    checking for AVCODEC... no
    configure: error: No package 'libavcodec' found
    No package 'libavutil' found. Pass --disable-avcodec to ignore this error.


To:


    checking for AVCODEC... no
    configure: error: Package requirements (libavcodec >= 57.37.100 libavutil >= 55.22.101) were not met:

    No package 'libavcodec' found
    No package 'libavutil' found

    Consider adjusting the PKG_CONFIG_PATH environment variable if you
    installed software in a non-standard prefix.

    Alternatively, you may set the environment variables AVCODEC_CFLAGS
    and AVCODEC_LIBS to avoid the need to call pkg-config.
    See the pkg-config man page for more details.
    make: *** [Makefile:1549: config.status] Error 1