pg_sendmail function compile problem

I added an include line but now it gives me the following error...

make
/bin/sh ./libtool --mode=compile
gcc -DHAVE_CONFIG_H -I. -I. -I.     -g -O2 -I/usr/local/pgsql/include -I/usr
/include/pgsql -c mail.c
mkdir .libs
gcc -DHAVE_CONFIG_H -I. -I. -I. -g -O2 -I/usr/local/pgsql/include -I/usr/inc
lude/pgsql -c  -fPIC -DPIC mail.c -o .libs/mail.lo
In file included from /usr/include/pgsql/c.h:47,
                 from /usr/include/pgsql/postgres.h:40,
                 from mail.c:29:
/usr/include/pgsql/config.h:662: os.h: No such file or directory
make: *** [mail.lo] Error 1

The os.h file points to  ../.././src/include/port/linux.h which does not
exist? :-(
I am showing my ignorance here I guess.

> "Joseph" <lters@mrtc.com> writes:
> > In file included from mail.c:29:
> > /usr/include/pgsql/utils/elog.h:37: parse error before
`CritSectionCount'
> > /usr/include/pgsql/utils/elog.h:37: warning: data definition has no type
or
> > storage class
>
> At a guess, this code is neglecting to include postgres.h first.  All
> of the internal headers in PG assume that you included postgres.h
> beforehand.
>
> regards, tom lane
>