Cygwin no longer provides the iconv command by default, which means the
translation of LIBDIR into OCAML_STDLIB_DIR cannot be guaranteed to
succeed. Technically, an unusual PREFIX could also result in an invalid
C constant in the Unix build.

Introduce a (relatively) small C program to carry out the function of
iconv and also translate the constant on the Unix side as well.

Eliminates the dependency on many invocations of `tr` and `cut` to a
single invocation of a C auxiliary.

On Windows, the C auxiliary is considerably faster than the large number
of calls to tr and cut which were necessary in stdlib/StdlibModules.