Made the cmake option for using SSE4 work with visual studio.

dlib/cmake

@@ -12,9 +12,9 @@ if (NOT TARGET dlib)
         add_definitions("-Wreturn-type")
     endif()
 
+    # Setup some options to allow a user to enable SSE and AVX instruction use.  
     option(USE_SSE4_INSTRUCTIONS "Compile your program with SSE4 instructions" OFF)
     option(USE_AVX_INSTRUCTIONS "Compile your program with AVX instructions" OFF)
-
     if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang" OR "${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
         option(USE_SSE2_INSTRUCTIONS "Compile your program with SSE2 instructions" OFF)
         if(USE_AVX_INSTRUCTIONS)
@@ -30,7 +30,12 @@ if (NOT TARGET dlib)
         if(USE_AVX_INSTRUCTIONS)
             add_definitions(/arch:AVX)
         elseif (USE_SSE4_INSTRUCTIONS)
-            add_definitions(/arch:SSE2)  # There isn't an /arch:SSE4 flag in visual studio.
+            # There isn't an /arch:SSE4 flag in visual studio.  But we can tell
+            # dlib to use those instructions anyway with the DLIB_HAVE_SSE3 and
+            # DLIB_HAVE_SSE41 #defines.
+            add_definitions(/arch:SSE2)  
+            add_definitions(-DDLIB_HAVE_SSE3)
+            add_definitions(-DDLIB_HAVE_SSE41)
         elseif(USE_SSE2_INSTRUCTIONS)
             add_definitions(/arch:SSE2)
         endif()