more MSVC cmake fixes

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b407d78..840ba6f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt

@@ -31,7 +31,7 @@
 
 if(CMAKE_COMPILER_IS_GNUCXX)
   if(CMAKE_SYSTEM_NAME MATCHES Linux)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wnon-virtual-dtor -Wno-long-long -ansi -Wundef -Wcast-align -Wchar-subscripts -Wall -W -Wpointer-arith -Wwrite-strings -Wformat-security -fno-exceptions -fno-check-new -fno-common -fstrict-aliasing")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wnon-virtual-dtor -Wno-long-long -ansi -Wundef -Wcast-align -Wchar-subscripts -Wall -W -Wpointer-arith -Wwrite-strings -Wformat-security -Wextra -fno-exceptions -fno-check-new -fno-common -fstrict-aliasing")
     if(NOT EIGEN_TEST_LIB)
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic")
     endif(NOT EIGEN_TEST_LIB)