disable blas if C++ compiler is MSVC

CMakeLists.txt

@@ -294,11 +294,13 @@ add_subdirectory(unsupported)
 
 add_subdirectory(demos EXCLUDE_FROM_ALL)
 
+if(NOT MSVC)
 if(EIGEN_LEAVE_TEST_IN_ALL_TARGET)
   add_subdirectory(blas)
 else()
   add_subdirectory(blas EXCLUDE_FROM_ALL)
 endif()
+endif(NOT MSVC)
 
 # must be after test and unsupported, for configuring buildtests.in
 add_subdirectory(scripts EXCLUDE_FROM_ALL)