File 0961-use-cpp20.patch of Package OpenBoard

From ffeea1b662b012bd25a025f2130fa2c2044919f9 Mon Sep 17 00:00:00 2001
From: Vekhir <Vekhir@yahoo.com>
Date: Mon, 13 May 2024 10:58:33 +0200
Subject: [PATCH] fix: Make CMAKE_CXX_STANDARD a cache variable

poppler 24.05 exposes std::string::starts_with in its headers
which requires C++20. Requiring C++20 means dropping support
for still maintained distributions. As such, the C++ standard
defaults to the current C++17, but can be overridden where
necessary.

Emit a status message showing the chosen C++ standard for debug
purposes.
---
 CMakeLists.txt | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 97ad4f50f..33a5599b2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -80,7 +80,7 @@ set(OPENBOARD_MIMEICON_FILE resources/linux/ch.openboard.application-ubz.svg)
 # Basic compiler settings
 # ==========================================================================
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to use - defaults to C++17")
 set(CMAKE_INCLUDE_CURRENT_DIR ON)
 set(CMAKE_AUTOMOC ON)
 set(CMAKE_AUTOUIC ON)
@@ -89,6 +89,8 @@ list(APPEND CMAKE_AUTOUIC_SEARCH_PATHS
     ${OPENBOARD_FORMS_DIR}
 )
 
+message(STATUS "Using C++" ${CMAKE_CXX_STANDARD})
+
 # OpenMP support
 include(FindOpenMP)