MinGW tweak: Build with -mthreads. 



git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@3368 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/common/configure.ac b/common/configure.ac
index d590e05..c07b7fd 100644
--- a/common/configure.ac
+++ b/common/configure.ac
@@ -28,6 +28,7 @@
 case "$host_os" in
     mingw*)
         CPPFLAGS="$CPPFLAGS -DWINVER=0x0500 -D_WIN32_IE=0x0500"
+        CXXFLAGS="$CXXFLAGS -mthreads"
         ;;
 esac