Summary: fix, make sure ssp actually works before using it

Summary: fix, make sure ssp actually works before using it
Keywords: bldchn diagnostics

This changeset hopefully resolves the issue Nelson had with ssp and
-fstack-protector.  Using crystal ball technology and rune readings
it turns out that gcc single-handedly adds -lssp as soon as
-fstack-protector is specified, regardless if it works or exists.

* m4/sxe-compiler.m4 (SXE_STACK_FLAGS): check if ssp works before adding
  -fstack-protector

Signed-off-by: Sebastian Freundt <hroptatyr@fresse.org>