Document that MSVC cannot be used to build shared libraries.
Binaries crash when they are linked to intl-8.dll. Whereas binaries
don't crash when merely linked to iconv-2.dll.
I have debugged this problem for several hours, and think it is a
problem either in libtool or in the Microsoft linker.
* INSTALL.windows: Mention to use --disable-shared with MSVC.
