package.snappy: fix static builds with threads

For some toolchains, libstdc++ uses pthread symbols:

    .../lib/libstdc++.a(eh_alloc.o): In function `__gnu_cxx::__mutex::lock()':
    eh_alloc.cc:(.text._ZN9__gnu_cxx7__mutex4lockEv[_ZN9__gnu_cxx7__mutex4lockEv]+0x2):
    undefined reference to `pthread_mutex_lock'

and a lot of other pthread symbols...

However, when doing a static build, there is no way for the linker to know
what library to link in, so the build fails miserably if -pthread is not
specified at link time.

Fixes:
    http://autobuild.buildroot.org/results/15e/15ecaa23f0116e8120b3d62e553c838f0303df35/
    http://autobuild.buildroot.org/results/f0a/f0abe301816e39eb4ae26d3e8cd42d90901d5ac5/



[Peter: only perform workaround if toolchain has threads support, tweak comment]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>