Add file name conversion from $build to toolchain.

Add file name conversion from $build to toolchain.

* configure.ac: Ensure to_tool_file_cmd is available to Makefile.
* libltdl/m4/libtool.m4 (_LT_PATH_CONVERSION_FUNCTIONS): Add
cache variable lt_cv_to_tool_file_cmd that describes how to
convert file names from $build to toolchain format.
* libltdl/config/ltmain.m4sh (func_to_tool_file): New function
that utilizes the above.
* Makefile.am: Ensure to_tool_file_cmd is included in
TEST_ENVIRONMENT so that it is passed to (old testsuite) tests.
* testsuite.at: Ensure to_tool_file_cmd is passed as a variable
setting on the configure line for (new testsuite) tests.
* doc/libtool.texi (libtool script content): Update with
to_tool_file_cmd description.
(Cygwin to MinGW Cross): Update to mention lt_cv_to_tool_file_cmd.

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