Merge pull request #961 from Vekhir/poppler-cpp20

fix: Enable use of C++20 for compatibility with poppler 24.05

Author: kaamui

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)