impdef, msvc++ 6, and GMP

[Kevin Ryde]

I'm going to add --output-def=.libs/libgmp-3.dll.def to the library
creation to get a .def file, and have the following in the manual,

     A MINGW DLL build of GMP can be used with Microsoft C.  Libtool
     doesn't install a `.lib' format import library, but it can be
     created with MS `lib' as follows and copied to the install
     directory.  Similarly for `libmp' and `libgmpxx'.

          cd .libs
          lib /def:libgmp-3.dll.def /out:libgmp-3.lib


The libgmp def comes out with spurious copies of the mingw isascii and
toascii, but that seems to be some dodginess in the mingw headers (not
inlining isascii), so I'm going to quietly ignore that.

The filename used to be .dll-def, but libtool has switched to .dll.def
for what it generates under the -export-symbols option, so at the risk
of breaking everyone's scripts I'm going to follow suit.

I wish libtool would do all this itself, but I guess MS
inter-operation isn't high on the list of priorities.  :-(