Smart --with-package=DIR (for absentminded users)

[Thomas Lavergne]

Hi all,

   Sorry if this is already in the archive. I looked through it and was 
not lucky. You must also know that I am quite new to autoconf but... I 
need to learn fast!

   I want to implement the --with-package=DIR option to my project's 
configure script. The --with-package[=yes|no] thing is straightforward 
using AC_ARG_WITH(). I have however some questions regarding others' 
experience wth the =DIR part.

1. What should I do if the user is so dumb to actually give a DIR which 
does not exist? IMHO, I should issue an AC_MSG_ERROR and terminate the 
configuration: we cannot configure taking into account the user's input. 
What do you think of that and is there a consensus on how to handle such 
a situation.

2. How can I detect that /path/to/foo helps in any way for accessing 
foo.h and libfoo.a? Specifically, if library foo is available in the 
system directories (e.g. /usr/include and /usr/lib), then providing 
--with-foo=yes and --with-foo=/wrong/path/to/foo will have the same 
effect. For foo is accessible to the compiler and linker even if the 
options -I and -L point to 'wrong' directories, i.e directories not 
containing foo.h and libfoo.a. Is there a (portable) way of detecting 
that? In my case, the user might have a custom-made version of foo (with 
the same file names) and want to use it. I agree he should not have 
given a wrong path to foo in the first place but users are users and I 
want to avoid that the software is configured and compiled with the 
'system-version' of the library when the user (thought he) especially 
demanded to use another version.
Any (portable or not) idea?

Thanks in advance.
Thomas