configure: Allow host fragments to react to --enable-host-shared.

This makes the host_shared value available to host makefile
fragments.

It uses this to adjust Darwin's mdynamic-no-pic in the case that
shared host resources are required.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

config/
	* mh-darwin: Require a non-shared host configuration to
	enable  mdynamic-no-pic where that is supported.