configure: When host-shared, pass --with-pic to in-tree lib configs.

If we are building PIC/PIE host executables, and we are building dependent
libs (e.g. GMP) in-tree those libs need to be configured to generate PIC code.

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

	* Makefile.def: Pass host_libs_picflag to host dependent library
	configures.
	* configure.ac (host_libs_picflag): New configure variable set to
	'--with-pic' when building 'host_shared'.