Won't build on Illumos

@ptribble ever see this one before?

```
[root@tommy-dd378711 ~/cstore_fdw]# make
gcc -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -m64 -fPIC --std=c99 -I. -I./ -I/opt/postgresql/11.1/include/server -I/opt/postgresql/11.1/include/internal -c -o cstore_fdw.o cstore_fdw.c
In file included from /opt/postgresql/11.1/include/server/postgres.h:47:0,
from cstore_fdw.c:18:
/opt/postgresql/11.1/include/server/utils/elog.h:318:20: error: unknown type name 'sigjmp_buf'
extern PGDLLIMPORT sigjmp_buf *PG_exception_stack;
^
: recipe for target 'cstore_fdw.o' failed
make: *** [cstore_fdw.o] Error 1

```