pkg-autotools.mk: default host AUTORECONF{,_OPT} to the target values

When we have patches touching configure.ac and hence need to set
<pkg>_AUTORECONF = YES we also have to remember to set
HOST_<pkg>_AUTORECONF = YES if we build both host and target versions,
which is often forgotten (latest case was bison).

Fix it by making the host versions of _AUTORECONF and _AUTORECONF_OPT
default to whatever the target versions are set to, similar to how we
handle a number of the other variables.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>