cmake check_variable_exists() examples

HDFGroup/hdf5 config/ConfigureChecks.cmake :350 

CHECK_VARIABLE_EXISTS (CLOCK_MONOTONIC HAVE_CLOCK_MONOTONIC)

InsightSoftwareConsortium/ITK Modules/ThirdParty/HDF5/src/itkhdf5/config/cmake/ConfigureChecks.cmake :358 

CHECK_VARIABLE_EXISTS (CLOCK_MONOTONIC HAVE_CLOCK_MONOTONIC)

neovim/neovim cmake/FindLibIntl.cmake :55 

check_variable_exists(_nl_msg_cat_cntr HAVE_NL_MSG_CAT_CNTR)

TigerVNC/tigervnc CMakeLists.txt :214 

check_variable_exists(CLSID_VideoProcessorMFT HAVE_VIDEO_PROCESSOR_MFT)