[mingw|cygwin] Modify cwrapper to invoke target directly.

[mingw|cygwin] Modify cwrapper to invoke target directly.

* libltdl/config/ltmain.m4sh (func_to_native_path):
New function. If $host is mingw, and $build is mingw
or cygwin, convert path to mingw native format.
(func_to_native_pathlist): New function. Ditto, for
:-separated pathlists.
(func_emit_cwrapperexe_src) [__CYGWIN__ && __STRICT_ANSI__]:
Ensure putenv and setenv are declared. Define HAVE_SETENV.
(func_emit_cwrapperexe_src) [main]: Add new constants to
hold desired PATH settings; initialize and use functions
above to convert to $host format if necessary. Add new
command-line options --lt-env-set, --lt-env-prepend, and
--lt-env-append. No longer emit wrapper script as integral
part of launching child. Remove support for (now) unnecessary
$TARGETSHELL. Exec actual target executable directly.
(func_emit_cwrapperexe_src) [lt_setenv, lt_extend_str]
[lt_split_name_value, lt_opt_process_env_set]
[lt_opt_process_env_prepend, lt_opt_process_env_append]
[lt_update_exe_path, lt_update_lib_path]: New functions.