CMake: improve setting C++ standard and configuration summary

CMake/fl_add_library.cmake: set target_compile_features(.. cxx_std_NN)
  which is inherited by consumers of the library.
  This prevents CMake user projects from failing if they would
  otherwise compile with an older C++ standard.

CMake/fl_debug_var.cmake: add INTERFACE_COMPILE_FEATURES to fl_debug_target()

CMake/fl_summary.cmake: update summary format for longer variable names

CMakeLists.txt:
  - check CMAKE_CXX_STANDARD given by the user (commandline)
    and set C++ standard related CMake variables in the CMake cache.
    The latter is experimental: setting in the cache may be reverted.
  - add C++ standard related info to configure summary.

CMake/fl_add_library.cmake

@@ -2,7 +2,7 @@
 # Macro used by the CMake build system for the Fast Light Tool Kit (FLTK).
 # Originally written by Michael Surette
 #
-# Copyright 1998-2024 by Bill Spitzak and others.
+# Copyright 1998-2025 by Bill Spitzak and others.
 #
 # This library is free software. Distribution and use rights are outlined in
 # the file "COPYING" which should have been included with this file.  If this
@@ -98,7 +98,10 @@ function(fl_add_library LIBNAME LIBTYPE SOURCES)
 
   target_compile_definitions(${TARGET_NAME} PRIVATE "FL_LIBRARY")
 
-  # Set PUBLIC include and linker directories
+  # Set PUBLIC properties, e.g. C++ standard and include and linker directories.
+  # These properties are inherited by consumers of the libraries
+
+  target_compile_features(${TARGET_NAME} PUBLIC "cxx_std_${CMAKE_CXX_STANDARD}")
 
   if(0) # DEBUG
     message(STATUS "fl_add_library and alias         : fltk::${alias_name} ALIAS ${TARGET_NAME}")

CMake/fl_debug_var.cmake

@@ -123,6 +123,7 @@ function(fl_debug_target name)
       LINK_LIBRARIES
       COMPILE_DEFINITIONS
       INTERFACE_COMPILE_DEFINITIONS
+      INTERFACE_COMPILE_FEATURES
       INTERFACE_INCLUDE_DIRECTORIES
       INTERFACE_LINK_DIRECTORIES
       INTERFACE_LINK_LIBRARIES)

CMake/fl_summary.cmake

@@ -30,7 +30,7 @@ include(${CMAKE_CURRENT_LIST_DIR}/fl_debug_var.cmake)
 ########################################################################
 
 macro(fl_summary title value)
-  fl_expand_name(label "${title}" 24)
+  fl_expand_name(label "${title}" 30)
   message(STATUS "${label} ${value}")
 endmacro(fl_summary title value)
 

CMakeLists.txt

@@ -46,16 +46,20 @@ project(FLTK VERSION 1.5.0)
 # Note 3: This code is experimental and intentionally undocumented.
 #         It may be removed or changed w/o notice.
 
-if(NOT DEFINED CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 11)
+if(DEFINED CMAKE_CXX_STANDARD)
+  if(CMAKE_CXX_STANDARD EQUAL 98 OR CMAKE_CXX_STANDARD LESS 11)
+    message(FATAL_ERROR           "The FLTK project requires at least C++11")
+  endif()
+else()
+  set(CMAKE_CXX_STANDARD          11 CACHE STRING "Minimal C++ Standard")
 endif()
 
 if(NOT DEFINED CMAKE_CXX_STANDARD_REQUIRED)
-  set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON CACHE BOOL "Minimal C++ Standard Required?")
 endif()
 
 if(NOT DEFINED CMAKE_CXX_EXTENSIONS)
-  set(CMAKE_CXX_EXTENSIONS OFF)
+  set(CMAKE_CXX_EXTENSIONS        OFF CACHE BOOL "Use Compiler Specific C++ Extensions")
 endif()
 
 #######################################################################
@@ -69,12 +73,24 @@ include(CMake/fl_add_library.cmake)
 # right now we don't use compatibility functions
 # include(CMake/compatibility.cmake)
 
-if(0)
+if(0) # debug most important CMake variables and features
+  # FLTK version
   fl_debug_var(FLTK_VERSION_MAJOR)
   fl_debug_var(FLTK_VERSION_MINOR)
   fl_debug_var(FLTK_VERSION_PATCH)
   fl_debug_var(FLTK_VERSION)
+  # CMake version and C++ standard
   fl_debug_var(CMAKE_VERSION)
+  fl_debug_var(CMAKE_CXX_STANDARD)
+  fl_debug_var(CMAKE_CXX_STANDARD_REQUIRED)
+  fl_debug_var(CMAKE_CXX_EXTENSIONS)
+  # Build platform
+  fl_debug_var(MSVC)
+  fl_debug_var(MINGW)
+  fl_debug_var(MSYS)
+  fl_debug_var(UNIX)
+  fl_debug_var(APPLE)
+  fl_debug_var(CMAKE_CROSSCOMPILING)
 endif()
 
 # Set FLTK_VERSION in the cache so user projects can access it,
@@ -345,7 +361,17 @@ else()
   fl_summary("${title}" "No")
 endif()
 
-fl_summary_yn("Use std::" FLTK_USE_STD)
+message("")
+
+if(FLTK_USE_STD)
+  fl_summary("Use std::"                    "Yes - obsolete: always enabled since FLTK 1.5.0")
+else()
+  fl_summary("Use std::"                    "No - *should* always be enabled since FLTK 1.5.0 !!")
+endif()
+
+fl_summary(   "CMAKE_CXX_STANDARD"          "${CMAKE_CXX_STANDARD}")
+fl_summary_yn("CMAKE_CXX_STANDARD_REQUIRED" CMAKE_CXX_STANDARD_REQUIRED)
+fl_summary_yn("CMAKE_CXX_EXTENSIONS"        CMAKE_CXX_EXTENSIONS)
 
 message("")
 message(STATUS "End of Configuration Summary --\n")