[PROJ] build failure with GCC 9.1.0

[Even Rouault]

> Yes, io.ccp. The patch applies above your pull request and change the CS
> class name in two templates

That's exactly what my yesterday pull request against master does:
https://github.com/OSGeo/PROJ/pull/1637/files

(perhaps you were mentionning the older commit from February
https://github.com/OSGeo/PROJ/commit/2f8bd934860b135044c5122e3272f7cc41ba81e7 
that is already in 6.2.0)


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