CODE128: Add minimal encodation algorithm (non-extended ASCII only)

from ZXing (props Alex Geller) - about 25-60% slower depending on
  data & stack heavy but does improve some outcomes when FNC1s
  present (GS1_MODE or manual) although not much else it appears
  (the previous algorithm was very good), but has a logical clarity
  the other lacked - funcs `c128_dxsmooth()` etc shared with
  CODE16K now moved there and renamed as `c16k_`;
  also fix extended char latching when exactly 4 extended chars
  at end
  also manual code set switching now honoured exactly even if
  immediate shift required;
manual: make explicit that AI "(00)" and "(01)" prefixes added by
  Zint are HRT-only
general: add `extern "C"` wrappers to a few header files

backend/pcx.h

@@ -35,7 +35,7 @@
 
 #ifdef __cplusplus
 extern "C" {
-#endif
+#endif /* __cplusplus */
 
 #ifdef OUT_USE_PRAGMA_PACK
 #pragma pack(1)
@@ -68,7 +68,7 @@ extern "C" {
 
 #ifdef __cplusplus
 }
-#endif
+#endif /* __cplusplus */
 
 /* vim: set ts=4 sw=4 et : */
 #endif /* Z_PCX_H */