Fix build error due to open_memstream

[Bruno Haible]

On Solaris 10, I get a build error:

Undefined                       first referenced
 symbol                             in file
open_memstream                      ../libpoke/.libs/libpoke.so
ld: fatal: symbol referencing errors. No output written to .libs/poke

This is because libpoke makes use of the unportable function 'open_memstream'.
This function exists in
  - Solaris 11.4, but not in Solaris <= 11.3 and Solaris OpenIndiana,
  - macOS 10.13, but not in macOS 10.5,
  - NetBSD 8.0, but not in NetBSD 7.1.1,
  - AIX 7.1, but not in AIX 6.1.
and is also missing in
  - HP-UX 11.31
  - Minix 3.3.

At least Solaris 11.3 is still supported for 3 more years, see
<https://en.wikipedia.org/wiki/Solaris_(operating_system)#Version_history> .

Gnulib provides a utility module 'string-buffer', as a replacement for
'open_memstream' (since today).

Here's a patch to make use of this module instead of 'open_memstream'.

The changes to the .c files in the test suite are needed to avoid a link
error in the test suite programs, caused by the following:
  - The new module relies on vsnprintf().
  - To avoid buggy vsnprintf outputs, I made it depend on 'vsnprintf-posix'
    (poke/bootstrap.conf already relies on 'printf-posix').
  - The redirection "#define vsnprintf rpl_vsnprintf" is only meant for
    libpoke, but is visible also in the test suite.
  - /usr/include/dejagnu.h has a couple of inline functions which reference
    vsnprintf. So, they see the redirection to rpl_vsnprintf.
  - The symbol rpl_vsnprintf is not exported from libpoke.so.
If you have a better idea how to fix this?

0001-Don-t-use-unportable-function-open_memstream.patch
Description: Text Data