Add a PGF codec version ID to be able to make rules with clien application...

Add a PGF codec version ID to be able to make rules with clien application accodingly with libpgf API changes.
Raphael, it's not possible to do it with PGFCodecVersion string and preprocessor. Of course both must be updated at each libpgf version.
We use already this way in libraw and libexiv2.
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