Directory where the standard headers were found?

Hi everyone,
i'm using AC_HEADER_STDC in my configure.in to check for the standard 
headers.... Now i want to check for other header files via 
AC_CHECK_HEADER(...) in the same directory the standard header files 
were found by AC_HEADER_STDC. Is there a possibility to get the 
directory where the standard header files were found?

regards,
Michael