config: make backtrace optional

config: make backtrace optional

When building DPDK with musl, there is need not to disable
backtrace to remove some references to execinfo.h which is
not supported by musl now.
This also applies to some other libc implementation which
doesn't support backtrace() and backtrace_symbols().

musl is an implementation of the userspace portion
of the standard library functionality described in
the ISO C and POSIX standards, plus common extensions.
Got more details about musl from http://www.musl-libc.org .

Signed-off-by: Wei Dai <wei.dai@intel.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>