execute_cmd.h is not included in installed headers

[Robert E. Griffith]

"execute_cmd.h" is not listed in the INSTALLED_HEADERS macro which 
determines which headers are included for the install-headers target.

Is it problematic for a loadable builtin to execute shell functions or 
is it maybe an oversight that that header is not included? If I build 
against the full source it appears to work fine but I wonder if there 
are edge cases that cause problems.

If my builtin should not use functions from execute_cmd.h, is there a 
safe way to invoke a shell function from a builtin?

--BobG