meson: build print backends

This changes the configure option into two states:
auto: build all that can be build (default)
A list of backend names: build them and fail if we can't

"papi" is missing because it's not in Debian and I can't test it.

modules/meson.build

@@ -1,8 +1,2 @@
-# TODO
-#if os_unix
-#  subdir('printbackends')
-#else
-  print_backends = []
-#endif
-
 subdir('input')
+subdir('printbackends')