ruby: do not overwrite RUBY_CONF_ENV

In commit 53bf889c ("ruby: should not
use dl_iterate_phdr() on Blackfin") we added
ac_cv_func_dl_iterate_phdr=no to RUBY_CONF_ENV when building on the
Blackfin architecture.

However, while doing so, we overwrote the previous value of
RUBY_CONF_ENV, which was defined to have custom CFLAGS:

RUBY_CONF_ENV = CFLAGS="$(RUBY_CFLAGS)"

This has no visible effect because the custom CFLAGS are only used on
SuperH/SuperH 64, while the overwrite of RUBY_CONF_ENV takes place on
Blackfin.

However, it doesn't look very pretty, and future additions to
RUBY_CONF_ENV may break this subtle behavior.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>