This avoids the error:

   ld: /home/mcournoyer/src/ring-project/daemon/contrib/x86_64-unknown-linux-gnu/lib/libswscale.a(swscale.o): relocation R_X86_64_PC32 against symbol `ff_M24A' can not be used when making a shared object; recompile with -fPIC
   ld: final link failed: bad value

caused by the missing '-Wl,-Bsymbolic' linker directive (which is
documented as required by ffmpeg when a shared library is built
against a static ffmpeg object).

* src/Makefile.am (libring_la_LDFLAGS): Set to AM_LDFLAGS.
[HAVE_WIN32]: Append using the += operator.
[HAVE_OSX]: Likewise.
[HAVE_LINUX]: Remove; it was not necessary.

Change-Id: Id2af9ea5812b2d1fd3b4f7f893108bb94671ffc6
GitLab: jami-packaging#101