7.4.6 build failures on Solaris 9 with Sun cc

Hi all, before I delve too deeply into this, has anyone else tried building
7.4.6 on Solaris 9 (sparc) ? I'm seeing build failures using Sun's cc:

make[4]: Entering directory `/tmp/postgresql-7.4.6/src/backend/access/heap'
cc -Xa -O -v -I../../../../src/include -c -o tuptoaster.o tuptoaster.c
"tuptoaster.c", line 831: member can not have variably modified type: data
"tuptoaster.c", line 937: cannot recover from previous errors
cc: acomp failed for tuptoaster.c

Which is:
struct
{
struct varlena hdr;
char data[TOAST_MAX_CHUNK_SIZE];
} chunk_data;

The compiler is cc: Sun C 5.6 2004/06/02 (Sun Studio 9)

If no-one's seen this before, I'll do some digging.

Regards, Philip.