cmake: Replace FindXerces with FindXercesC

Upstream renamed the macro to avoid confusion with the
Java implementation.
cpp: bioformats: Correct placement of warning pragmas in PixelProperties
Merge pull request #1467 from rleigh-dundee/cmake-xerces

cmake: Replace FindXerces with FindXercesC
Merge pull request #1469 from rleigh-dundee/cpp-pixelprop-warnings

cpp: bioformats: Correct placement of warning pragmas in PixelProperties