libtool: fix conversion warnings in cwrapper

build-aux/ltmain.in (func_emit_cwrapperexe_src:main): XMALLOC wants a
size_t. Also use int instead of intptr_t for the return value (which
is fine since the _spawnv call is synchronous).
(func_emit_cwrapper_src) [MSVC]: Remove the intptr_t helper define.
(func_emit_cwrapperexe_src:find_executable): Use size_t for variables
involved in strlen computations.
(func_emit_cwrapperexe_src:lt_setenv): Likewise.
(func_emit_cwrapperexe_src:lt_extend_str): Likewise.
(func_emit_cwrapperexe_src:lt_update_exe_path): Likewise.
THANKS: Update.

Signed-off-by: Yaakov Selkowitz <yselkowitz@users.sourceforge.net>
Signed-off-by: Peter Rosin <peda@lysator.liu.se>