From 42a04c064d4b31c3a85210311f3ada163c406a25 Mon Sep 17 00:00:00 2001
From: Albrecht Schlosser <albrechts.fltk@online.de>
Date: Fri, 7 Mar 2025 15:17:56 +0100
Subject: [PATCH] Improve setting minimal compiler standard (C++11)

... so the user can override all settings.

This may need more tweaks in the future.
---
 CMakeLists.txt | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index da59bcd46..97b57f714 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -35,8 +35,28 @@ cmake_minimum_required(VERSION 3.15.0 FATAL_ERROR)
 
 project(FLTK VERSION 1.5.0)
 
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+#######################################################################
+# set the required C++ standard -- may be overridden by the user
+#######################################################################
+
+# Note 1: We don't check if the user sets a standard older than C++11,
+#         but if they do, the build will fail.
+# Note 2: For optimal portability we disable compiler specific
+#         extensions, but this can also be overridden by the user.
+# 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)
+endif()
+
+if(NOT DEFINED CMAKE_CXX_STANDARD_REQUIRED)
+  set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
+endif()
+
+if(NOT DEFINED CMAKE_CXX_EXTENSIONS)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+endif()
 
 #######################################################################
 # include macro and function definitions for general usage
-- 
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