CMakeLists: drop CMAKE_MINIMUM_VERSION to 3.6

While we do need the compiler to support C++17 features, we can
get by on older GCC using CMake 3.6 and -std=c++-1z with some
other fixes.

CMAKE_CXX_STANDARD can be raised intelligently to 17 based on
whether we detect CMake 3.8 or higher.

CMakeLists.txt

@@ -1,5 +1,3 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
 # Visual Studio 2019 is supported from CMake 3.14.1
 
 # Tested generators:
@@ -42,7 +40,7 @@
 
 # "Visual Studio 16 2019" + LLVM 8.0 (clang) optional platform generator Win32 and x64
 
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 # VS2019: 3.14.1
 # Intel 2021: 3.20
 
@@ -84,9 +82,11 @@ if(NOT HEADERS_ONLY)
   set(CMAKE_CXX_STANDARD_LIBRARIES "" CACHE STRING "" FORCE)
 
   # We require C++17 or higher.
+if(CMAKE_VERSION VERSION_GREATER 3.7)
   set(CMAKE_CXX_STANDARD 17)
   set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
   set(CMAKE_CXX_EXTENSIONS FALSE)
+endif()
 
   # Detect Intel processors and turn Intel SIMD on or off automatically.
   message("-- Detected target processor as: ${CMAKE_SYSTEM_PROCESSOR}")
@@ -305,6 +305,19 @@ if(NOT HEADERS_ONLY)
 
   ELSE()
     # not MS Visual Studio IDE
+
+    # CMAKE_CXX_STANDARD doesn't cover the use-case of pre-final C++17 support,
+    # but I'd assume most setups with a new enough version of CMake to use
+    # CMAKE_CXX_STANDARD 17 would also be running a version of GCC/Clang new enough
+    # to not need this.  So this will most likely only ever be used by setups running
+    # older versions of CMake; regardless, it shouldn't be necessary to force a
+    # CMAKE_VERSION check on this part unless the mere presence of CMAKE_CXX_STANDARD 17
+    # ends up causing problems for the older compilers here.
+    if( ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU")   AND (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 8)) OR
+        ((CMAKE_CXX_COMPILER_ID STREQUAL "Clang") AND (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5)) )
+      set( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++1z" )
+    endif()
+
     if(ENABLE_INTEL_SIMD)
       SET( CMAKE_CXX_FLAGS  "${CMAKE_CXX_FLAGS} -msse2 -DINTEL_INTRINSICS" )
     endif()

avs_core/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 if (WIN32)
   set(CoreName "AviSynth")

plugins/ConvertStacked/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 set(PluginName "ConvertStacked")
 set(ProjectName "Plugin${PluginName}")

plugins/DirectShowSource/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 set(PluginName "DirectShowSource")
 set(ProjectName "Plugin${PluginName}")

plugins/ImageSeq/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 set(PluginName "ImageSeq")
 set(ProjectName "Plugin${PluginName}")

plugins/Shibatch/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 set(PluginName "Shibatch")
 set(ProjectName "Plugin${PluginName}")

plugins/TimeStretch/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 set(PluginName "TimeStretch")
 set(ProjectName "Plugin${PluginName}")

plugins/TimeStretch/SoundTouch/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 # Find all the input files
 FILE(GLOB SoundTouch_Sources RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}"

plugins/VDubFilter/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 set(PluginName "VDubFilter")
 set(ProjectName "Plugin${PluginName}")

plugins/VFAPIFilter/CMakeLists.txt

@@ -1,6 +1,4 @@
-# We need CMake 3.8 at least, because we require
-# CMAKE_CXX_STANDARD to be set to C++17.
-CMAKE_MINIMUM_REQUIRED( VERSION 3.8.2 )
+CMAKE_MINIMUM_REQUIRED( VERSION 3.6.2 )
 
 set(PluginName "VFAPIFilter")
 set(ProjectName "Plugin${PluginName}")