usage of ac_check_header

[Joao Miguel Ferreira]

Hello all,

I'm not being able to use AC_CHECK_HEADER to check if the my system has
some headers installed:

I've tried all these approaches:

---------------------------------------------------------
AC_CHECK_HEADER(xpath.h,,AC_MSG_ERROR([cannot find headers for
libxml2]))
-------------------------------------------------------
AC_CHECK_HEADER(libxml2/libxml/xpath.h libxml/xpath.h
xpath.h,,AC_MSG_ERROR([cannot find headers for libxml2]))
-----------------------------------------------------
AC_CHECK_HEADER(libxml/xpath.h,,AC_MSG_ERROR([cannot find headers for
libxml2]))
-------------------------------------

but ./configure always tells me the same:

...
checking xpath.h usability... no
checking xpath.h presence... no
checking for xpath.h... no
configure: error: cannot find headers for libxml2

-------------------
but I am quite sure the headers are there (I installed them: libxml and
libxml-dev;); see bellow

$ ls -la /usr/include/libxml2/libxml/ | grep xpath.h
-rw-r--r-- 1 root root 16042 2006-10-26 22:23 xpath.h

--------------------

What am I doing wrong... this smells like another $PATH_TO_SOMETHING
issue... but I really don't know what it is ???

I'm able to use AC_CHECK_LIB very fine, though. It simply works well
out-of-the-box ! What's diferent ??

++thx;

Joao