Set ERL_LIBS for building/installing the docs
    
    Without setting ERL_LIBS, the "make release-docs" target
    cannot find the include_lib("xmerl/lib/xmerl.erl") file
    and thus the doc build and install fails.
    
    We should examine adding the release-docs target to the
    "make docs" command in the %build section, instead of
    having release-docs being implicitly invoked by the
    install-docs target in the %install section.