SConstruct : Update C++ version to C++17

Also remove explicit override in CI setup, so we inherit the version from the SConstruct.

Author: johnhaddon

.github/workflows/main.yml

@@ -156,11 +156,11 @@ jobs:
 
     - name: Build
       run: |
-       scons -j 2 install BUILD_TYPE=${{ matrix.buildType }} OPTIONS=${{ matrix.options }} CXXSTD=c++14 BUILD_CACHEDIR=sconsCache
+       scons -j 2 install BUILD_TYPE=${{ matrix.buildType }} OPTIONS=${{ matrix.options }} BUILD_CACHEDIR=sconsCache
 
     - name: Test
       run: |
-        scons ${{ matrix.tests }} BUILD_TYPE=${{ matrix.buildType }} OPTIONS=${{ matrix.options }} CXXSTD=c++14 BUILD_CACHEDIR=sconsCache
+        scons ${{ matrix.tests }} BUILD_TYPE=${{ matrix.buildType }} OPTIONS=${{ matrix.options }} BUILD_CACHEDIR=sconsCache
       env:
         IECORE_RTLD_GLOBAL: 0
 

SConstruct

@@ -93,7 +93,7 @@ o.Add(
 o.Add(
 	"CXXSTD",
 	"The C++ standard to build against.",
-	"c++11"
+	"c++17"
 )
 
 o.Add(