build: fix static dll link for mingw

dll's are no longer dynamically linked to libgcc/libstdc++/libssp

configure.ac

@@ -226,6 +226,15 @@ case $host in
        *) AC_MSG_ERROR("Could not determine win32/win64 for installer") ;;
      esac
      AC_SUBST(WINDOWS_BITS)
+
+     dnl libtool insists upon adding -nostdlib and a list of objects/libs to link against.
+     dnl That breaks our ability to build dll's with static libgcc/libstdc++/libssp. Override
+     dnl its command here, with the predeps/postdeps removed, and -static inserted. Postdeps are
+     dnl also overridden to prevent their insertion later.
+     dnl This should only affect dll's.
+     archive_cmds_CXX="\$CC -shared \$libobjs \$deplibs \$compiler_flags -static -o \$output_objdir/\$soname \${wl}--enable-auto-image-base -Xlinker --out-implib -Xlinker \$lib"
+     postdeps_CXX=
+
      ;;
   *darwin*)
      TARGET_OS=darwin