Fix build with icu >=75

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -53,13 +53,13 @@ if ( SLOP_OPENGL )
 endif()
 
 set_property(TARGET ${LIBRARY_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
-set_property(TARGET ${LIBRARY_NAME} PROPERTY CXX_STANDARD 11)
+set_property(TARGET ${LIBRARY_NAME} PROPERTY CXX_STANDARD 17)
 set_property(TARGET ${LIBRARY_NAME} PROPERTY CXX_EXTENSIONS OFF)
 
 add_executable(${EXECUTABLE_NAME} src/main.cpp)
 
 set_property(TARGET ${EXECUTABLE_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
-set_property(TARGET ${EXECUTABLE_NAME} PROPERTY CXX_STANDARD 11)
+set_property(TARGET ${EXECUTABLE_NAME} PROPERTY CXX_STANDARD 17)
 set_property(TARGET ${EXECUTABLE_NAME} PROPERTY CXX_EXTENSIONS OFF)
 
 set( CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/modules" )
