Windows build warnings

Author: Dmitry Yashunin

CMakeLists.txt

@@ -50,7 +50,7 @@ if(HNSWLIB_EXAMPLES)
     elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
       SET( CMAKE_CXX_FLAGS  "-Ofast -lrt -std=c++11 -DHAVE_CXX0X -march=native -fpic -w -fopenmp -ftree-vectorize -ftree-vectorizer-verbose=0" )
     elseif (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
-      SET( CMAKE_CXX_FLAGS  "-Ofast -lrt -std=c++11 -DHAVE_CXX0X -openmp -march=native -fpic -w -fopenmp -ftree-vectorize" )
+      SET( CMAKE_CXX_FLAGS  "/O2 -DHAVE_CXX0X /openmp" )
     endif()
 
     # examples