pkg-config is broken after HDF5 detection rewriting

[q-posev]

Calling pkg-config --libs trexio after make install returns the following:

Package @PKG_HDF5@ was not found in the pkg-config search path.
Perhaps you should add the directory containing `@PKG_HDF5@.pc'
to the PKG_CONFIG_PATH environment variable
Package '@PKG_HDF5@', required by 'trexio', not found

The new HDF5 detection macro does not define PKG_HDF5 so it is not propagated to the trexio.pc