configure: use the poppler-data required version in pkg-config check.
This makes no functional differences since this package has a pkg-config file only since 0.4.7, our current requirement. This means that searching for any poppler-data through pkg-config is the same as searching for this minimum version. But at least that makes for better output in case of error. People who wish to build GIMP will have proper error information of the minimum requirement for poppler-data.
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