Fix CMake 3.17 STRING warning

CMake Warning (dev) at CMakeLists.txt:29 (set):
  implicitly converting 'string' to 'STRING' type.
This warning is for project developers.  Use -Wno-dev to suppress it.

CMakeLists.txt

@@ -17,16 +17,16 @@ if(APPLE)
 			# set the architecture for iOS 
 			if (${IOS_PLATFORM} STREQUAL "OS")
 				set (IOS_ARCH armv6 armv7 armv7s arm64)
-				set (CMAKE_OSX_ARCHITECTURES ${IOS_ARCH} CACHE string  "Build architecture for iOS")
+				set (CMAKE_OSX_ARCHITECTURES ${IOS_ARCH} CACHE STRING  "Build architecture for iOS")
 			else (${IOS_PLATFORM} STREQUAL "OS")
 				set (IOS_ARCH x86_64)
-				set (CMAKE_OSX_ARCHITECTURES ${IOS_ARCH} CACHE string  "Build architecture for iOS Simulator")
+				set (CMAKE_OSX_ARCHITECTURES ${IOS_ARCH} CACHE STRING  "Build architecture for iOS Simulator")
 			endif (${IOS_PLATFORM} STREQUAL "OS")
 
 		else(IOS)
 			# set the architectures for OS X
 			set (OSXI_ARCH x86_64)
-			set (CMAKE_OSX_ARCHITECTURES ${OSXI_ARCH} CACHE string  "Build architecture for OS X universal binaries")		
+			set (CMAKE_OSX_ARCHITECTURES ${OSXI_ARCH} CACHE STRING  "Build architecture for OS X universal binaries")
 		endif(IOS)
 	endif (NOT CMAKE_OSX_ARCHITECTURES AND BUILD_UNIVERSAL_BINARIES)
 endif(APPLE)