Extend libtool dll namespaces for mingw-w64

[JonY]

Hi,

Currently, on Win32 platforms, Cygwin uses the "cyg" prefix for dlls,
and MinGW based systems uses the "lib" prefix.

This works fine, until mingw-w64 showed up with 64bit dlls. This
problem is especially apparent with trying to build mingw-w64 cross
compilers.

For example, both mingw and mingw-w64 builds libstdc++-6.dll from GCC.
When installed, there might be up to 3 incompatible versions of
libstdc++-6.dll, from mingw.org, 32bit mingw-w64 and 64bit mingw-w64.

I suggest the following naming scheme.

mingw.org:      lib<name>-<major>.dll (unchanged)
Cygwin:         cyg<name>-<major>.dll (unchanged)
mingw-w64(64):  lib64<name>-<major>.dll
mingw-w64(32):  lib32<name>-<major>.dll

libtool should also check if GCC "-m32" or "-m64" is used, and select
the proper namespace accordingly (mingw-w64 GCC can do multilib).

Comments?