Create a .def file when building with MinGW

Preferably we'd tell CMake to make it itself instead of specifiying compiler-
specific flags, but there doesn't seem to be a way to.

CMakeLists.txt

@@ -1040,6 +1040,9 @@ SET_TARGET_PROPERTIES(${LIBNAME} PROPERTIES VERSION ${LIB_VERSION}
 IF(WIN32 AND NOT LIBTYPE STREQUAL "STATIC")
     SET_TARGET_PROPERTIES(${LIBNAME} PROPERTIES PREFIX "")
 ENDIF()
+IF(MINGW)
+    SET_TARGET_PROPERTIES(${LIBNAME} PROPERTIES LINK_FLAGS "-Wl,--output-def,${LIBNAME}.def")
+ENDIF()
 
 TARGET_LINK_LIBRARIES(${LIBNAME} common ${EXTRA_LIBS})