alleges to document Oracle Solaris 11.4 and appears to support execvpe. Yes,
it's listed in Linux pages as a GNU extension and it's not on the Open Group page for the exec family, but symmetry makes it quite an obvious extension. Perhaps that could be detected by a configure test.
The Linux page for vfork, but not the Open Group one, says that vfork suspend the caller until the exec is done, so the caller could restore environ after. I don't immediately see that it could hurt and should fix it for Linux.
Or there's copying the environment somewhere else. Huh, even just the environ pointer.