Keep frame pointers if using gcc frame intrinsics.

Specify -fno-omit-frame-pointer when using __builtin_frame_address() and
__builtin_return_address() for backtracing.  This fixes backtracing
failures on e.g. i686 for optimized builds.

configure.ac

@@ -784,6 +784,7 @@ fi
 )
 if test "x$backtrace_method" = "x" -a "x$enable_prof_gcc" = "x1" \
      -a "x$GCC" = "xyes" ; then
+  JE_CFLAGS_APPEND([-fno-omit-frame-pointer])
   backtrace_method="gcc intrinsics"
   AC_DEFINE([JEMALLOC_PROF_GCC], [ ])
 else