The sources should reach files in the $builddir using #include "".
Besides, it is not guarranteed to be around:

    CC       shared/clients_cli_nmcli-nm-setting-metadata.o
  cc1: error: ./clients/cli: No such file or directory [-Werror=missing-include-dirs]
  cc1: all warnings being treated as errors
  Makefile:12971: recipe for target 'shared/clients_cli_nmcli-nm-setting-metadata.o' failed