Simple

Add the following line to your Makefile

HGVERSION:= $(shell hg parents --template 'hgid: {node|short}'

and

-DHGVERSION="\"${HGVERSION}\""

to your CPPFLAGS. Now one can use the define HGVERSION inside the code.

Rebuild files using HGVERSION

Make rebuilds targets only if one of the dependencies is never. That means even if the version has changed, due to a change somewhere, files using HGVERSION will not be rebuild. To overcome this add new dependency in the Makefile to to these files:

version.cc: hgstamp
hgstamp: update_hgstamp
update_hgstamp:
        [ -f hgstamp ] || touch hgstamp
        echo $(HGVERSION) | cmp -s hgstamp - || echo $(HGVERSION) > hgstamp

In this example version.cc depend has the extra dependency hgstamp, which is updated by the rule 'update_hgstamp', if and only if the version has realy has changed.