explicitly require CXX_STANDARD=23, mainly for abseil

CMakeLists.txt

@@ -7,6 +7,11 @@ project(
         LANGUAGES CXX
 )
 
+# Required by Kuiper Engine, but needs to be explicitly specified for Abseil, at least
+set(CMAKE_CXX_STANDARD 23)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
 add_library(
         kuiper
         STATIC
@@ -30,7 +35,13 @@ add_library(
         src/renderer/gl_renderer.cpp
         src/window/input_system.cpp
 )
-target_compile_features(kuiper PRIVATE cxx_std_23)
+
+target_compile_features(kuiper PUBLIC cxx_std_23)
+if(CXX_STANDARD LESS 23)
+    message(FATAL_ERROR
+        "Kuiper Engine requires CXX_STANDARD >= 23 (got: ${CXX_STANDARD})")
+endif()
+
 target_include_directories(kuiper PUBLIC "include/")
 
 # Compiler options
@@ -191,6 +202,8 @@ FetchContent_Declare(
 FetchContent_MakeAvailable(flecs)
 
 # Abseil C++
+set(ABSL_BUILD_TESTING OFF CACHE BOOL "" FORCE)
+
 FetchContent_Declare(
         abseil
         GIT_REPOSITORY https://github.com/abseil/abseil-cpp.git