cmake: set HAVE_LIBZBD before creating "acconfig.h"

cmake: set HAVE_LIBZBD before creating "acconfig.h"

`acconfig.h` is generated using

configure_file(
  ${CMAKE_SOURCE_DIR}/src/include/config-h.in.cmake
  ${CMAKE_BINARY_DIR}/include/acconfig.h
)

in `config-h.in.cmake`, the cmake variable of `HAVE_LIBZBD` is checked.
so we need to ensure that this variable is visible from this
`configure_file()` statement.

Signed-off-by: Kefu Chai <kchai@redhat.com>