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

* src/Makefile.am (extra_ltoptions): New.
(libgpgme_la_LDFLAGS): Use it.
(libgpgme_pthread_la_LDFLAGS): Ditto.
(libgpgme_glib_la_LDFLAGS): Ditto.

Since gcc 4.8 there is a regression in Mingw64 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).

Note that we don't use this patch for the Qt version which, as C++
programs, actually requires that DLL,

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