[libredwg] contributions v2 available

[Thien-Thi Nguyen]

See:

 http://www.gnuvola.org/wip/  (libredwg)

 The v2 series aims to enable (optionally configured) runtime trace
 control.  It is inspired by a comment on the LibreDWG wiki.

Details:

 f7a9d0a [build] Move config.h into src/.

   It's nice to have all the source components together in one
   directory.  Also, this allows Automake to avoid generating "-I..".

 f9b1a87 [build] Make every .c file #include "config.h".

   This propagates the information found by the configure script to the
   source files.  Our immediate need is to pass the fact that the option
   "--enable-trace" was specified.  However, looking ahead, there may be
   other kinds of information that will be useful, such as endianness,
   alloca availability, word size, etc, and gnulib is a nice way to add
   support for such things, so we make sure this change is forward
   compatible with gnulib.

 54455cc [build] Add preliminary "configure --enable-trace" support.

   This uses AC_ARG_ENABLE, defaulting to "no".

 c3e29a8 [build r] Add runtime tracing support for read (decode) path.
 32bf5b3 [build w] Add runtime tracing support for write (encode) path.

   These make the respective file-static `loglevel' variables in
   src/{de,en}code.c subject to a lookup of the env var LIBREDWG_TRACE.
   The lookup is done only once.  If there is no tracing, the default
   is now hardcoded as zero (no output).

 fb534d3 Refill README; nfc.
 2a91cbc [admin] Mention --enable-trace in README; nfc.

   This documents the configure script feature.  It includes a blurb:
   "NB: This is an experimental feature that is not in its final form."

   That's because i think "levels" of tracing is not the best; i
   personally would prefer to see a bitset approach: The library-global
   (as opposed to bifurcated file-static) variable `trace' would hold a
   set of independent flags that enable tracing for various aspects of
   the operation.  For example: encoding, decoding, top-level objects,
   strings, etc.

   Another idea is to support a file descriptor for trace output.

   Together, these features would be very helpful for "make check".
   WDYT?