Move CPP standard to C++17 (#1712)

Trivial change to move CPP standard to C++17 for v1.39. As agreed on by the MaterialX TSC.

CMakeLists.txt

@@ -6,7 +6,7 @@ set(MATERIALX_LIBRARY_VERSION ${MATERIALX_MAJOR_VERSION}.${MATERIALX_MINOR_VERSI
 
 # Cmake setup
 cmake_minimum_required(VERSION 3.16)
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
 set(CMAKE_MACOSX_RPATH ON)
 enable_testing()