libunwind: needs dynamic library support

libunwind configure script explicitly links libunwind against
libgcc_s. libgcc_s is only guaranteed to be available for toolchains
that supports dynamic linking: pure static linking toolchains only
have libgcc.a, not libgcc_s.so.

Therefore, let's make libunwind unavailable on toolchains that lack
dynamic linking support. We could potentially support linking with
libgcc, but switching to libgcc_s was done upstream because libgcc was
lacking some symbols on ARM
(https://lists.nongnu.org/archive/html/libunwind-devel/2014-06/msg00024.html). Even
though recent gcc versions seem to provide such symbols in libgcc.a,
having libunwind available on static linking configurations is not a
useful enough use-case to do the necessary research to find when this
issue was fixed in gcc.

Since libunwind is not used as a mandatory dependency in any package,
adding this !BR2_STATIC_LIBS dependency is trivial and nicely avoids
the problematic situation.

This fixes two different autobuilder failures:

 - Gstreamer 1.x programs failing to link, because libunwind links
   against libgcc_s that isn't available (static linking
   configuration):

   http://autobuild.buildroot.net/results/9d4fbf7167e9afce0eef5c9e0cfd42c966ecba36/

 - Gmrender-resurrect, which fails to link, because GStreamer 1.x uses
   some libunwind functionality, but does not take into account the
   libunwind dependency in its .pc files (static linking
   configuration):

   http://autobuild.buildroot.net/results/0a3a2485c187a000482c178f1e9c64dd716a858f/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

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