bug#22342: Infinite loop when GUILE_FOR_BUILD == guile

[Peter TB Brett]

I ran across an interesting problem while cross-compiling Guile 2.0.11 
with MinGW.

"GEN guile-procedures.texi" sat in an infinite loop, due to meta/guile 
exec-ing itself in a tight loop.

This was because I had set GUILE_FOR_BUILD to "guile" in my configure 
invocation.  In the uninstalled_env environment, "guile" is 
"meta/guile", not my installed version of guile in my PATH.

It might be helpful for configure to insist that GUILE_FOR_BUILD is an 
absolute path in order to avoid this happening to other naive 
cross-compiling users!

                                  Peter