nxcompshad/{configure.ac,src/Makefile.am}: only enable new ELF dtags if target system actually is ELF-based.

Same method as used in the previous commit, but easier done thanks to
autotools and libtool.

Fixes: ArcticaProject/nx-libs#610

 This code can be reactivated any time in the future once
 some potential maintainer turns up and makes this work
 on the MS Windows / Cygwin platform.