m4: readline: Extract code setting -D_FUNCTION_DEF
m4: readline: Extract code setting -D_FUNCTION_DEF

The current code is a bit awkward, and we're going to need
to share it later anyway. We can drop the call to AC_SUBST()
while we're at it, since LIBVIRT_CHECK_LIB() already marks
READLINE_CFLAGS for substitution.

The new code goes to some extra length to avoid setting
-D_FUNCTION_DEF twice: this is mostly for cosmetic reasons,
and it's necessary because LIBVIRT_CHECK_READLINE() is called
twice: once on its own, and then once more as part of
LIBVIRT_CHECK_BASH_COMPLETION().

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

jfehlig/libvirt,crobinso/libvirt,eskultety/libvirt,jfehlig/libvirt,crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,olafhering/libvirt,nertpinx/libvirt,eskultety/libvirt,zippy2/libvirt,zippy2/libvirt,jardasgit/libvirt,andreabolognani/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,zippy2/libvirt,nertpinx/libvirt,eskultety/libvirt,fabianfreyer/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,andreabolognani/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,zippy2/libvirt,eskultety/libvirt,eskultety/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,andreabolognani/libvirt,jardasgit/libvirt,libvirt/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,olafhering/libvirt,andreabolognani/libvirt,crobinso/libvirt