problems with AC_CHECK_HEADERS

[dkouroun]

Hi all!
I am reading the autoconf manual and I
try to understand where AC_CHECK_HEADERS
searches to find the headers.

***Yes it does search. But WHERE and how can I change the
   default search location???***

I have problems running configure
and stuff like that in my FreeBSD.
While it is working fine under SuSE
it cannot find GL/glut.h in checkheaders
AC_CHECK_HEADERS(GL/glut,, e.t.c)

in Linux it can find it easily!
It is located in /usr/include/GL/glut.h
In FreeBSD it is located in 
/usr/X11R6/include/GL/glut.h

Can you please help me with that?
Can anybody give me a sample configure.in for the following
case where I:


1:) want to include some system libraries which are
     not in the same directories for different OS.
     e.g. Suppose I use FreeBSD and Linux but
     the opengl libraries are located in different
     directories. What macros should I add in configure
     such it searches for that libraries and when it finds
     them it automatically adds -L/dir/ -llib1, -llib2...

2:) how can I do the same for headeer files?

Best Wishes!
Drosos.