Setting external directory in ./configure --options

[Xochitl Lunde]

I'm not having any luck finding this in the autoconf manual.

I need to compile and link in my embedded OS, but I want to choose the 
top-level path to the OS because I have different versions.  The embedded OS 
has its own make system that does not use autoconf.  (So I wouldn't use 
AC_CONFIG_SUBDIRS.)  In my configuration script, I have a top-level directory 
variable defined "ac_os_dir=/cygdrive/c/os_dir" and many path variables and 
strings built from ac_os_dir such as: ac_os_includes, ac_os_ldscripts, 
ac_os_boothdr, ac_os_image_header, ac_os_binpack, etc.  The OS compiles into 
libraries using its own makefiles, but to link the final installation image I 
need to have all the paths to the linker scripts and image packager.

I want to set the 'ac_os_dir' on the configure command line.  I see 
AC_ARG_ENABLE and AC_ARG_WITH both sort of expect the value to be "yes" or 
"no", and not a directory.  I tried AC_ARG_WITH(myos, ...) and then used 
-with-myos and -myos-libraries=/cygdrive/c/libpath  but these were not 
understood. (I thought it might work like -x-libraries=path)

I am pretty sure AC_ARG_VAR will work, but is this at all the right way to do 
it?  What I most want is something like: ./configure -ospath=/cygdrive/c/path

Customers will never install using this configure.  The configure is for 
developers only.