debug/ltrace: fix LDFLAGS for libsupc++ and libiberty

The configure script correctly detects libsupc++ and libiberty, but in
the linker stage it tries to link in both libraries without taking care
of the test result.

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
[yann.morin.1998@anciens.enib.fr: rework patch depth to be -p1]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Remy Bohmer"

 reporter_url="linux@bohmer.net"

 reporter_comment="Example for building a toolchain for m68000 bare metal targets"