Fix official Windows build

(broken by c170b3a)

Setting a maximum version in cmake_minimum_required() effectively sets
the behavior to NEW for all policies introduced in all CMake versions up
to and including that maximum version.  The NEW behavior for CMP0091,
introduced in CMake 3.15, uses CMake variables to specify the MSVC
runtime library against which to link, rather than placing the relevant
flags in CMAKE_C_FLAGS*.  Thus, replacing /MD with /MT in CMAKE_C_FLAGS*
no longer has any effect when using CMake 3.15+.  Because the TurboJPEG
libraries in the official libjpeg-turbo Windows packages always use /MT,
this caused the official Windows TurboVNC build to fail.

win/CMakeLists.txt

@@ -9,14 +9,18 @@ if(NOT MSVC)
 endif()
 
 # Use the static C library for all build types
-foreach(var CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
-	CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
-	CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
-	CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
-	if(${var} MATCHES "/MD")
-		string(REGEX REPLACE "/MD" "/MT" ${var} "${${var}}")
-	endif()
-endforeach()
+if(CMAKE_VERSION VERSION_EQUAL "3.15" OR CMAKE_VERSION VERSION_GREATER "3.15")
+	set(CMAKE_MSVC_RUNTIME_LIBRARY MultiThreaded)
+else()
+	foreach(var CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
+		CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
+		CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
+		CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)
+		if(${var} MATCHES "/MD")
+			string(REGEX REPLACE "/MD" "/MT" ${var} "${${var}}")
+		endif()
+	endforeach()
+endif()
 
 foreach(var CMAKE_CXX_FLAGS CMAKE_CXX_FLAGS_DEBUG CMAKE_CXX_FLAGS_RELEASE
 	CMAKE_CXX_FLAGS_MINSIZEREL CMAKE_CXX_FLAGS_RELWITHDEBINFO)