devel/libiberty breaks lang/{gcc46,gcc47,gcc48} builds (Was: alloca.c build failures for gcc46, gcc47 and gcc48 on netbsd-6 amd64)

[David Brownlee <abs%absd.org@localhost>]

devel/libiberty, which appears to be from a relatively ancient version
of gcc (3.4.0), breaks lang/gcc4{6,7,8} builds. Specifically it the
version of libiberty.h is used in preference to the one bundled with
gcc.

The offending line command was

 /var/obj/pkg/lang/gcc46/work/obj/./prev-gcc/xgcc
-B/var/obj/pkg/lang/gcc46/work/obj/./prev-gcc/
-B/usr/pkg/gcc46/x86_64-unknown-netbsd6.1./bin/
-B/usr/pkg/gcc46/x86_64-unknown-netbsd6.1./bin/
-B/usr/pkg/gcc46/x86_64-unknown-netbsd6.1./lib/ -isystem
/usr/pkg/gcc46/x86_64-unknown-netbsd6.1./include -isystem
/usr/pkg/gcc46/x86_64-unknown-netbsd6.1./sys-include    -c
-DHAVE_CONFIG_H -g -O2 -gtoggle
-I/var/obj/pkg/lang/gcc46/work/.buildlink/include -I/usr/pkg/include
-I/usr/include -I.
-I/var/obj/pkg/lang/gcc46/work/gcc-4.6.4/libiberty/../include  -W
-Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -pedantic
-fpic /var/obj/pkg/lang/gcc46/work/gcc-4.6.4/libiberty/alloca.c -o
pic/alloca.o

and the error (due to a missing XNEWVEC definition)

/var/obj/pkg/lang/gcc46/work/gcc-4.6.4/libiberty/alloca.c:200:43:
error: expected expression before 'char'

Obviously the value of an ancient libiberty.h is somewhat
questionable, but it might make sense for a gcc/build guru to take a
poke at the gcc build config to make it more robust...