In dbus/config.m4:
if test "$PHP_DBUS" != "no"; then
  if test "$PHP_LIBXML" = "no"; then
    AC_MSG_ERROR([DBUS extension requires LIBXML extension, add --enable-libxml])
  fi

In dbus/dbus.c:
static const zend_module_dep dbus_deps[] = {
    ZEND_MOD_REQUIRED("libxml")
    ZEND_MOD_END
};  

zend_module_entry dbus_module_entry = {
    STANDARD_MODULE_HEADER_EX,
    NULL,
    dbus_deps,
    "dbus",                     /* extension name */
...
There is also ZEND_MOD_CONFLICTS and ZEND_MOD_OPTIONAL; as well as ZEND_MOD_REQUIRED_EX and ZEND_MOD_OPTIONAL_EX