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/filemem.h

@@ -33,6 +33,10 @@
 #ifndef Z_FILEMEM_H
 #define Z_FILEMEM_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
 #include <stdio.h>
 #include "common.h"
 
@@ -91,5 +95,9 @@ INTERNAL int fm_error(struct filemem *restrict const fmp);
    NOTE: don't use, included only for libpng compatibility */
 INTERNAL int fm_flush(struct filemem *restrict const fmp);
 
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+
 /* vim: set ts=4 sw=4 et : */
 #endif /* Z_FILEMEM_H */