Adjust naming of MSVC import libraries.

Adjust naming of MSVC import libraries.

* libltdl/m4/libtool.m4 (_LT_SYS_DYNAMIC_LINKER) [mingw, cygwin]
[pw32, cegcc] <cl*, library_names_spec>: Adjust naming of the
import library from foo-<major-version>.lib to foo.dll.lib, to
avoid clashes with the static library for unversioned libraries.
(_LT_LINKER_SHLIBS, _LT_LANG_CXX_CONFIG) [mingw, cygwin, pw32]
[cegcc] <cl*, archive_expsym_cmds>: Explicitely name the import
library to match the above.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>