Fix: detect_cpu requires c++20

Author: dancasarin

CMakeLists.txt

@@ -282,10 +282,9 @@ if (KFR_ARCH IN_LIST DETECT_NAMES AND NOT CMAKE_CROSSCOMPILING)
         ${CMAKE_CURRENT_SOURCE_DIR}/cmake/detect_cpu.cpp
         CMAKE_FLAGS
             "-DINCLUDE_DIRECTORIES=${CMAKE_CURRENT_SOURCE_DIR}/include"
-            -DCMAKE_CXX_STANDARD=17
+            -DCMAKE_CXX_STANDARD=20
             -DCMAKE_CXX_STANDARD_REQUIRED=ON
             "-DCOMPILE_DEFINITIONS=-DCPU_${KFR_ARCH}=1"
-            -DCMAKE_CXX_EXTENSIONS=ON
         COMPILE_OUTPUT_VARIABLE COMPILE_OUT
         RUN_OUTPUT_VARIABLE RUN_OUT)
     if (COMPILE_RESULT AND RUN_RESULT EQUAL 0)

docs/cxxdox.yml

@@ -33,7 +33,7 @@ input:
     compile_options:
       - "-DKFR_ENABLE_FLAC=1"
       - "-DKFR_FORCE_GENERIC_CPU=1"
-      - "-std=gnu++17"
+      - "-std=gnu++20"
       - "-DDOCUMENTATION"
 
 repository: https://github.com/kfrlib/kfr/blob/{TAG}/include/kfr/{FILE}#L{LINE}

docs/docs/dft2.md

@@ -29,5 +29,5 @@ The KFR implementation of the FFT:
 * has special implementations for FFT sizes up to 1024
 * has no external dependencies
 * is thread-safe, with no global data
-* is written in modern C++17
+* is written in modern C++20
 * is open source (GPL v2+ license, commercial license is availalble for closed source projects, see https://kfr.dev )