tools/nolibc: also mention how to build by just setting the include path

Now that a few basic include files are provided, some simple portable
programs may build, which will save them from having to surround their
includes with #ifndef NOLIBC. This patch mentions how to proceed, and
enumerates the list of files that are covered.

A comprehensive list of required include files is available here:

  https://en.cppreference.com/w/c/header

Signed-off-by: Willy Tarreau <w@1wt.eu>