Failed to compile on OSX Sierra 10.12

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ModestEx failes to compile on OSX Sierra 10.12 because of missing open_memstream.

[ 19%] Linking C executable modest_worker
Undefined symbols for architecture x86_64:
  "_open_memstream", referenced from:
      _eterm_vec_join in eterm_vec.c.o
      _eterm_to_string in eterm_vec.c.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [modest_worker] Error 1
make[1]: *** [CMakeFiles/modest_worker.dir/all] Error 2
make: *** [all] Error 2

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// memstream functions are conforming to POSIX.1-2008. These functions are
// not specified in POSIX.1-2001 and are not widely available on other
// systems.
https://www.gnu.org/software/gnulib/manual/html_node/open_005fmemstream.html

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For a quick solution I added open_memstream implementation from FreeBSD to this with a flag for APPLE machines.
This is obviously not the best solution, but it works. Better would be to get rid of using this function in the first place. I developed this package on ubuntu a couple of years ago and didnt realize that OSX lacks support for that.
Also, during investigating this, I realised that travis lacks support for build tests on osx system with erlang/erlixir. I tried several approaches to test this package properly on travis/osx and eventually gave up. So, the reason why this open_memstream slipped in the first place, is probably that I was relying on travis and didnt see that travis only tests against linux distros.
So, here you go...