On some distributions, libbfd is only available as static library. E.g.
on OpenSuse Leap 42.2, there are these two files:
libbfd.a
libbfd-2.28-system.so
backward-cpp, when attempting to link to bfd, will select the static
library and fail b/c there are undefined references to symbols from
libiberty/libz:
```
/usr/lib64/gcc/x86_64-suse-linux/4.8/../../../../lib64/libbfd.a(elflink.o):
In function `elf_link_add_object_symbols':
/home/abuild/rpmbuild/BUILD/binutils-2.26.1/build-dir/bfd/../../bfd/elflink.c:4733:
undefined reference to `objalloc_free_block'
/home/abuild/rpmbuild/BUILD/binutils-2.26.1/build-dir/bfd/../../bfd/elflink.c:5027:
undefined reference to `_sch_istable'
...
```
After this patch we also link against the direct dependencies of libbfd.a, namely libiberty and libz
Also see: https://github.com/KDAB/GammaRay/issues/386