Convert POSIX file names to toolchain format for MSVC

Convert POSIX file names to toolchain format for MSVC

* libltdl/m4/libtool.m4 (_LT_LINKER_SHLIBS, _LT_LANG_CXX_CONFIG)
[cygwin, mingw, pw32, cegcc] <cl*, postlink_cmds>: Make
sure the -outputresource: file name for the manifest tool is
in w32 format.
<cl*, archive_expsym_cmds>: Likewise for the MSVC command file
containing the exports and the output file name.
* libltdl/config/ltmain.m4sh (func_mode_link): Precalculate the
toolchain format of the output_objdir file name and store it in
the tool_output_objdir variable. Also replace any occurrence of
@TOOL_OUTPUT@ in postlink_cmds with the toolchain version of the
@OUTPUT@ content.
* doc/libtool.texi (libtool script content): Update description
of postlink_cmds to mention @TOOL_OUTPUT@.

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