MFC r312507:

Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This simplifies pathing in make/displayed output

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Don't install private libexecinfo headers

libexecinfo compatibility with devel/libexecinfo port

1. Match shlib number
2. Add libelf dependency

Suggested by: bapt[1]

Add libexecinfo Makefile

Sponsored by: DARPA, AFRL