bootstrap/7090: libstdc++ configure script fails to find install-sh when cross-compiling

[Joe Buck]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7090

Scott, if you use a different srcdir and builddir, can you successfully 
build
a cross-compiler?

If you can, then I'd like to downgrade the priority of this PR to 
"medium" so it
is no longer a blocker for the 3.2.1 release.  I'll add an item in the 
release
notes warning of the problem (if it is not fixed in time).