Configure zlib with --enable-host-shared for shared bfd
When bfd is configured as a shared library, we need to configure zlib
with --enable-host-shared since zlib is used by bfd.
PR ld/18355
* Makefile.def: Add extra_configure_flags to host zlib.
* configure.ac (extra_host_zlib_configure_flags): New. Set
to --enable-host-shared When bfd is to be built as shared
library. AC_SUBST.
* Makefile.in: Regenerated.
* configure: Likewise.