Check for empty architectures before setting -arch flag.

CMake/cotire.cmake

@@ -410,9 +410,11 @@ function (cotire_get_target_compile_flags _config _language _directory _target _
 		if (NOT _architectures)
 			get_target_property(_architectures ${_target} OSX_ARCHITECTURES)
 		endif()
-		foreach (_arch ${_architectures})
-			list (APPEND _compileFlags "-arch" "${_arch}")
-		endforeach()
+		if (_architectures)
+			foreach (_arch ${_architectures})
+				list (APPEND _compileFlags "-arch" "${_arch}")
+			endforeach()
+		endif (_architectures)
 		if (CMAKE_OSX_SYSROOT)
 			if (CMAKE_${_language}_SYSROOT_FLAG)
 				list (APPEND _compileFlags "${CMAKE_${_language}_SYSROOT_FLAG}" "${CMAKE_OSX_SYSROOT}")