What is "ac_nonexistent.h"?

[杨勇勇]

Hi, guys.
I'm new to Autoconf.
Recently I wrote an very, very simple example of using autotools. The example 
worked OK(all worked OK, including generated Makefiles). But when I added macro 
AC_CHECK_HEADERS([...]) and it went well to generate configure shell script.
So I ran ./configure, it's OK! Still I checked config.log, and found there is 
an error showing:
"error: ac_nonexistent.h, no such file or directory."
...
"Compilation terminated."
Fortunately that is the only error which appeared several times in the 
config.log file.
 
I tried the same example on both Fedora 14 and RHEL 6.0, similar situations 
were encountered.
 
So, can someone explain what is going on.
Someone people said that ac_nonexistent.h error is expected when developers use 
AC_CHECK_HEADERS() macro if the target header files do exist. Is that true?
 
Best wishes.