[cmake] Do not set CMAKE_CXX_STANDARD in JlCxxConfig.cmake

[thelfer]

JlCxxConfig.cmake sets CMAKE_CXX_STANDARD to 17 which makes it incompatible with project using newer standards (in my case the MFrontGenericInterfaceSupport: https://github.com/thelfer/MFrontGenericInterfaceSupport/)

One workaround is to do this ugly hack:

# julia-bindings (placed before compiler detection)
option(enable-julia-bindings "enable julia bindings support" OFF)
if(enable-julia-bindings)
  # The current version (0.16.0) of CxxWrap overrides the
  # CMAKE_CXX_STANDARD variables, so we reset it after
  # calling find_package to the current version
  set(_CMAKE_CXX_STANDARD "${CMAKE_CXX_STANDARD}")
  find_package(JlCxx REQUIRED)
  set(CMAKE_CXX_STANDARD "${_CMAKE_CXX_STANDARD}")
endif(enable-julia-bindings)