Unix build: make sure $(GDAL_INCLUDE) is first to avoid being confused by GDAL headers of a previous version elsewhere in the include path (#5664)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27976 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_vc12