Using backtrace() is of no use when building with PIE (which most
distro compilers do by default) and prevents catching the coredump
for later retracing, which is needed since distros usually don't
install debug symbols by default either.
This patch thus only enables backtrace() when --enable-maintainer-mode
is passed and also tries to explicitly disable PIE.
GLIBC is not necessarily the only library that provides execinfo.
With libexecinfo execinfo can be used also in other Libraries.
The patch lets the configure check the existence of the header
and the libexecinfo Library and uses them if avaible.
(also fixes compilation if execinfo is not avaible)
