when the encoding and decoding are the same is only allowed when the
decoding was legal. Also, use a VRT function to concatenate the
STRING_LIST into workspace (rather than rolling our own).

which a workspace is available

in the process, simplify identity decoding

- en-/decoding implementation details in separate sources
- enforce the en-/decode interfaces with typedefs

- establish a common interface for encoders and decoders, and
  re-implement the existing functions accordingly
- encoders always convert STRING_LIST to BLOB, decoders vice versa
- VMOD encode() and decode() write the new data to workspace
- transcode() writes the temporary BLOB to stack, then encodes it
  into a STRING in workspace
- if the same encoding is specified in transcode() for encode and decode,
  just return the concatenated STRING
  - XXX: except this won't reject illegal decodings
- simplify hex_encode(), and fix hex_en-/decode_l() (was backwards)
- add a macro to skip over empty and undefined strings in a STRING_LIST

- implement base64_encode(), no test yet
- added encodings BASE64URL and BASE64URLNOPAD

better error handling, fixes for parallel make, remove generated files from dist

On getting dependencies right:

See automake doc 9.4.1 Built Sources Example / Recording Dependencies manually

Conflicts:
	configure.ac

define PKG_CHECK_VAR if it doesn't exist