configure: PTHREADS should be defined on cygwin, as well

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Signed-off-by: Brian Paul <brianp@vmware.com>

(cherry picked from commit c1a4f249f1)

configure.ac

@@ -92,6 +92,9 @@ linux*|*-gnu*|gnu*)
 solaris*)
     DEFINES="$DEFINES -DPTHREADS -DSVR4"
     ;;
+cygwin*)
+    DEFINES="$DEFINES -DPTHREADS"
+    ;;
 esac
 
 dnl Add flags for gcc and g++