erlang: pass erl_xcomp_sysroot to configure

This is required for building the 'crypto', 'odbc', 'ssh'
and 'ssl' Erlang applications. These apps are skipped
regardless of whether they're enabled and their dependencies
can be met if this environment variable isn't set.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>