MSVC doesn't support the -l option, instead it expects the exact
library file name. Retain the -l option as long as possible as
libtool recognizes -l internally. Then, as late as possible
transform the -l option to an exact file name (-lfoo -> foo.lib).
This patch is against Libtool 2.2

• libltdl/m4/libtool.m4: Add tag variable dashl_xform which

specifies how to transform -l options for the linker.

• libltdl/config/ltmain.m4sh (func_mode_link): Transform -l

options using dashl_xform right before creating the
program or library.