Expression that either returns the output string from a program, or a specific string, if the program doesn't exist

Question

I'm using mercurial as a SCM, and an output from the hg parents command in my makefile to store build number and version information in my program. However, mercurial is not always present on machines where I try to build the program. Thus, hg parent fails.
I'd like to use a substitute string (hard-coded or output from other program) when mercurial is not available. But I'm no good in makefile scripting.
Can you provide a hint how to compose a makefile-command that would store the output from a hg parents if it's available, or output from date if hg is not available, in the internal variable.

Answer 1

This is bit cranky but it works for me:

X=$(shell hg parent || date)