FindHDF5: fix compiler detection when HL is disabled

A valid HDF5 installation with the "high level" extensions not
configured will *fail* to be correctly detected by CMake since
commit d9e39f3f89 (FindHDF5: check that compiler wrapper can
compile a minimal program, 2020-02-10, v3.18.0-rc1~744^2~1):

```
/.../hdf5/cmake_hdf5_test.c:2:10: fatal error: 'hdf5_hl.h' file not found
```

This does not stop the configuration but does prevent flags and
libraries from being recognized.
1 file changed