message: Check for process_vm_readv using CheckSymbolExists
  Instead of checking if code compiles, I think it is a better to check
  if the symbol exists. This is simpler and should do the same thing.
diff: @@ -14,14 +14,8 @@ check_cxx_symbol_exists(accept4 "sys/socket.h" HAVE_ACCEPT4)
  check_include_file(termios.h HAVE_TERMIOS_H)
  check_include_files("sys/types.h;sys/event.h" HAVE_SYS_EVENT_H)
  -check_cxx_source_compiles("
  - #include <sys/uio.h>
  - int main() { process_vm_readv(0, nullptr, 0, nullptr, 0, 0); return 0; }"
  - HAVE_PROCESS_VM_READV)
  -check_cxx_source_compiles("
  - #include <sys/syscall.h>
  - int main() { return __NR_process_vm_readv; }"
  - HAVE_NR_PROCESS_VM_READV)
  +check_cxx_symbol_exists(process_vm_readv "sys/uio.h" HAVE_PROCESS_VM_READV)
  +check_cxx_symbol_exists(__NR_process_vm_readv "sys/syscall.h" HAVE_NR_PROCESS_VM_READV)
  check_library_exists(compression compression_encode_buffer "" HAVE_LIBCOMPRESSION)
  