Minor bug fix in configure
Improve portability configure's tests
Handle non-shared build of Sundials/C

The resulting Sundials/ML will not be loadable from the toplevel (unless
you make a custom toplevel).
Update intro.doc

- Make note about --enable-shared.
- Reorder some paragraphs by priority.
- Hint in the installation instructions to look at caveats when
  compiling the library manually.
- Properly capitalize and punctuate platform and package names.