MINOR: tools: do not attempt to use backtrace() on linux without glibc

MINOR: tools: do not attempt to use backtrace() on linux without glibc

The function is provided by glibc. Nothing prevents us from using our
own outside of glibc there (tested on aarch64 with musl). We still do
not enable it by default as we don't yet know if all archs work well,
but it's sufficient to pass USE_BACKTRACE=1 when building with musl to
verify it's OK.

(cherry picked from commit 7caf073faa6962de15a18dadcaf200df95ce7889)
Signed-off-by: Willy Tarreau <w@1wt.eu>