libbacktrace does not respect --disable-werror

https://bugs.gentoo.org/667104
--- a/libbacktrace/configure
+++ b/libbacktrace/configure
@@ -11634,3 +11634,3 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 if test -n "${with_target_subdir}"; then
-  WARN_FLAGS="$WARN_FLAGS -Werror"
+  WARN_FLAGS="$WARN_FLAGS"
 fi
--- a/libbacktrace/configure.ac
+++ b/libbacktrace/configure.ac
@@ -138,3 +138,3 @@ ACX_PROG_CC_WARNING_OPTS([-W -Wall -Wwrite-strings -Wstrict-prototypes \
 if test -n "${with_target_subdir}"; then
-  WARN_FLAGS="$WARN_FLAGS -Werror"
+  WARN_FLAGS="$WARN_FLAGS"
 fi