[bug#27355] [PATCH 3/7] gnu: Add libsigrokdecode.

[Danny Milosavljevic]

Hi Theodoros,
Hi Hartmut,

the README lists some more dependencies:

 - doxygen (optional, only needed for the C API docs)
 - graphviz (optional, only needed for the C API docs)

Do you think it makes sense to build the API docs?

Also, reading srd.c it seems that this library embeds Python.  Is it still 
supposed to be a propagated-input then?  It doesn't need the executable at 
runtime or anything.  Users probably do need the Python standard library, so I 
guess it's OK...  Hartmut, what do you think?

Extra Python scripts for libsigrokdecode can be installed by the user into 
${XDG_DATA_HOME-.local/share}/libsigrokdecode/decoders which is nice.

Additionally, there's an environment variable SIGROKDECODE_DIR too.