Okay, I've had a bit of time to look (again) at gmake. This patch won't make it in its current form. It does only implement a very partial subset of what gmake does, namely the export VAR=VALUE syntax.
I'll have to check gmake's code more closely, because the other export forms DO expand the full line, not just the variable value. I'm pretty sure it's more dangerous to recognize export partly and not support the other forms. There's also the fact that it's not a usual bsd-make construct (introduced as a known dot form). There's usually some risk of false positives, stuff where lines may start with export but not be export directives...
