HOST_CC for windows DLL cross-compile

[Kevin Ryde]

I tried using the Debian packaged mingw32 cross-compiler to build a
windows DLL with cvs libtool but didn't at first realize I had to set
HOST_CC manually.  Perhaps libtool could probe for a build system
compiler itself when it needs one (for impgen.c).

I don't have an actual patch to suggest for this, but there's a chunk
of code in config.guess which ought to be right and I suppose the
HOST_CC setting then ought then be emitted into the libtool script.
Oh, and I guess CC_FOR_BUILD is the new name for this too.