x11/slop: Bump C++ version to C++17

Required to fix build with ICU 76.1

Reference: https://github.com/naelstrof/slop/issues/172

Approved by:	portmgr (blanket)

x11/slop/files/patch-CMakeLists.txt

+--- CMakeLists.txt.orig	2025-01-19 10:48:24 UTC
++++ CMakeLists.txt
+@@ -53,13 +53,13 @@ set_property(TARGET ${LIBRARY_NAME} PROPERTY CXX_STAND
+ 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" )