w32: Use -static-libgcc to avoid linking to libgcc_s_sjlj-1.dll.

* src/Makefile.am (extra_ltoptions): New.
(libassuan_la_LDFLAGS): Use it.

Since gcc 4.8 there is a regression in that plain C programs may link
to libgcc_s.a which has a dependency on libgcc_s_sjlj.dll. This is
for example triggered by using long long arithmetic on a 32 bit
Windows (e.g symbol __udivdi3).

This patch does not change anything right now but we add it anyway
in case long long will ever be used.

• Signed-off-by: Werner Koch <wk@gnupg.org>