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

* src/Makefile.am (extra_ltoptions): New.
(libgcrypt_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).

As usual the gcc maintainers don't care about backward compatibility
and declare that as some kind of compatibility fix and not as
regression from 4.7 and all earlier versions.

(cherry picked from commit 40a7bdf50e19faaf106470897fed72af623adc50)

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