Can't find ac_nonexistent.h

Have slogged this far: CPP (exists and current with Slackware 13.0 and gcc 4.3.3). config.log says:

configure:4840: gcc -E  conftest.c    <-- works thru to end of CPP and stops ok!
conftest.c:10:28: error: ac_nonexistent.h: No such file or directory
configure:4847: $? = 1
configure: failed program was:
| /* confdefs.h.  */

Ran

find / -name "*ac_nonexistent.h*"

and found it NOT.
Is this a "dummy" header? Where should ac_nonexistent.h be located? Where can I buy one? <);^)
Is there a way to tell configure to ignore this test??? Should I even do that?

My second problem in configure is:

configure:3653: checking for strerror in -lcposix
configure:3688: gcc -o conftest -g -O2   conftest.c -lcposix   >&5
/usr/lib/gcc/i486-slackware-linux/4.3.3/../../../../i486-slackware-linux/bin/ld: cannot find -lcposix
collect2: ld returned 1 exit status
configure:3695: $? = 1
configure: failed program was:

There is NO cposix to be detected by find.
Can't tell if these are problems in Slackware or in configure or maybe my ship has pulled out??
Will appreciate assistance.

According to this:
Random List of Packaging Problems - Confluence
it is intended to not exist hence its name!

Tony, many thanks. Really appreciate the URL! Did I miss an explanation of "cposix"?
If not I'll leave that open.

Sorry should have said that I was not answering the cposix part of the question!