How to avoid caching in [certain cases]?

[Alexey Morozov]

Hi everybody!

I have the following question regarding autoconf.

I tried to write a macro for a home-made library, that should
perform as follows:

1. Check for the header, say mylib.h
2. if the header is found, it's Ok, otherwise try to [temporarily] add
$MYLIB_HOME/include to the headers path and try to locate mylib.h again.
3. Do some other checks for the mylib

I used the straight but probably wrong way:

saved_CPPFLAGS=$CPPFLAGS
AC_CHECK_HEADER([mylib.h],
 [HAVE_MYLIB_H=yes],
 [
  CPPFLAGS="$CPPFLAGS -I$MYLIB_HOME/include"
  AC_CHECK_HEADER([mylib.h],.....
....

But I found that once mylib.h is checked, any other checks for mylib.h
are performed via cache, skipping real preprocessing, compiling etc. Is
there a way to avoid caching in this case? 'unset' is marked as
non-portable so I can't just add

unset ac_cv_header_mylib_h

after each AC_CHECK_HEADER. Or maybe there's a better way to solve the
problem (I still don't want to include $MYLIB_HOME/include by default if
it's possible).

Thank you in advance.

Alexey Morozov