Makescript now detects different avlib versions

Makescript now detects different avlib versions by grepping through the
version file of avlib. If the version is 53, it sets a compiler variable
(AV53), which is used to #define "aliases" for avcodec members, which have
been renamed in newer versions.

Note that, if the AV53 variable is not exported, the "standard" avlib names
(meaning version 54 or higher) will be used:

    * AVPixelFormat (53: PixelFormat)
    * AV_PIX_FMT_RGBA (53: PIX_FMT_RGBA)