cmake FOLLY_HAVE_ELF examples

facebook/folly CMake/folly-deps.cmake :174 

CHECK_INCLUDE_FILE_CXX(elf.h FOLLY_HAVE_ELF)

facebook/folly CMake/folly-deps.cmake :183 

message(STATUS "Setting FOLLY_HAVE_ELF: ${FOLLY_HAVE_ELF}")