Use CMAKE_CXX_STANDARD rather than hardcoding

Author: jstone-lucasfilm

src/testshade/CMakeLists.txt

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # https://github.com/AcademySoftwareFoundation/OpenShadingLanguage
 
-set (LLVM_COMPILE_FLAGS ${LLVM_COMPILE_FLAGS} "-std=c++17")
+set (LLVM_COMPILE_FLAGS ${LLVM_COMPILE_FLAGS} -std=c++${CMAKE_CXX_STANDARD})
 
 # The 'testshade' executable
 set ( testshade_srcs