So, I did this:

make distclean
./configure --with-pgpkglibdir=/usr/lib/postgresql/8.3/lib/
--with-pgconfigdir=/usr/bin/pg_config
--with-pgincludeserverdir=/usr/include/postgresql/8.3/server/
make

and got this:

gcc -g -O2 -Wall -Wmissing-prototypes -Wmissing-declarations -fpic
-I../.. -I/usr/include/postgresql/
-I/usr/include/postgresql/8.3/server  -c -o xxid.o xxid.c
In file included from /usr/include/postgresql/8.3/server/access/tupdesc.h:18,
                 from /usr/include/postgresql/8.3/server/utils/rel.h:17,
                 from /usr/include/postgresql/8.3/server/storage/bufmgr.h:18,
                 from /usr/include/postgresql/8.3/server/storage/bufpage.h:17,
                 from /usr/include/postgresql/8.3/server/access/relscan.h:18,
                 from /usr/include/postgresql/8.3/server/access/heapam.h:18,
                 from /usr/include/postgresql/8.3/server/executor/spi.h:26,
                 from xxid.c:20:
/usr/include/postgresql/8.3/server/catalog/pg_attribute.h:44: error:
expected ')' before numeric constant
/usr/include/postgresql/8.3/server/catalog/pg_attribute.h:152:
warning: data definition has no type or storage class
/usr/include/postgresql/8.3/server/catalog/pg_attribute.h:152:
warning: type defaults to 'int' in declaration of
'FormData_pg_attribute'
/usr/include/postgresql/8.3/server/catalog/pg_attribute.h:167: error:
expected '=', ',', ';', 'asm' or '__attribute__' before '*' token
/usr/include/postgresql/8.3/server/catalog/pg_attribute.h:252: error:
expected ')' before '(' token
/usr/include/postgresql/8.3/server/catalog/pg_attribute.h:253: error:
expected ')' before '(' token
/usr/include/postgresql/8.3/server/catalog/pg_attribute.h:254: error:
expected ')' before '(' token

and several thousand more lines of error / warnings after that.

Then this:

In file included from /usr/include/postgresql/8.3/server/utils/rel.h:17,
                 from /usr/include/postgresql/8.3/server/storage/bufmgr.h:18,
                 from /usr/include/postgresql/8.3/server/storage/bufpage.h:17,
                 from /usr/include/postgresql/8.3/server/access/relscan.h:18,
                 from /usr/include/postgresql/8.3/server/access/heapam.h:18,
                 from /usr/include/postgresql/8.3/server/executor/spi.h:26,
                 from xxid.c:20:
/usr/include/postgresql/8.3/server/access/tupdesc.h:72: error:
expected specifier-qualifier-list before 'Form_pg_attribute'
/usr/include/postgresql/8.3/server/access/tupdesc.h:85: error:
expected declaration specifiers or '...' before 'Form_pg_attribute'

I got errors like that from lots of different files.