netcdf-cxx package: set ldflags for hdf5 and netcdf (#11026)

Ensure that netcdf-cxx uses the Spack-installed hdf5 and netcdf4
rather than system-installed versions.

var/spack/repos/builtin/packages/netcdf-cxx/package.py

@@ -35,4 +35,11 @@ def configure_args(self):
             # There is no clear way to set this via configure, so set the flag
             # explicitly
             args.append('CPPFLAGS=-DUSE_NETCDF4')
+        # Add these to LDFLAGS explicitly, so the linker doesn't accidentally
+        # use system versions
+        ldflags = [
+            self.spec['netcdf'].libs.search_flags,
+            self.spec['hdf5'].libs.search_flags,
+        ]
+        args.append('LDFLAGS=' + ' '.join(ldflags))
         return args