While building latest release of Nexedi ERP5 (based on tag 1.0.463), I
encountered the following error on `hdf5` build:

```
-- Could NOT find ZLIB (missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR)
CMake Error at CMakeFilters.cmake:198 (message):
   ZLib support in HDF5 was enabled but not found
Call Stack (most recent call first):
  CMakeLists.txt:965 (include)
```

The patch applied is to simply add both environment variables that are mentioned
in the error message.

Related-Commit: nexedi/slapos@544b5f04
Reviewed-on: nexedi/slapos!1985
Reviewed-by: @jerome