HAVE_BACKTRACE gets set on systems with libunwind however
execinfo.h may be missing (on *BSD or musl for example).
HAVE_EXECINFO_H has been defined by the build system for a long
time but previously not used.
Signed-off-by: Randy Palamar <randy@rnpnr.xyz>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1207>