Re: various headers reporting "present but cannot be compiled"

[Ludovic Courtès]

Hi,

Yesterday, 11 hours, 7 minutes, 58 seconds ago, Chris Tracy wrote:
> Any idea what is up with this?  I get a few "present but cannot be
> compiled" errors when running configure before compiling zebra (0.94
> or 0.95pre1) on a FreeBSD 4.9 system.

If you look at Autoconf 2.59's manual in the `Generic Header Checks'
node:

     Previous versions of Autoconf merely checked whether the header was
  accepted by the preprocessor.  This was changed because the old test was
  inappropriate for typical uses.  Headers are typically used to compile,
  not merely to preprocess, and the old behavior sometimes accepted
  headers that clashed at compile-time.  If you need to check whether a
  header is preprocessable, you can use `AC_PREPROC_IFELSE' (*note
  Running the Preprocessor::).

So, in your case, this typically means that the header in question was
found (ie. the preprocessor did not fail) but then it was impossible to
compile a simple C file including it (maybe because some other required
header was not included).  Looking at your `config.log' file may give
you clues.

Ludovic.