Darwin, jit: Fix build [PR100613].

Darwin, jit: Fix build [PR100613].

The generic unix build is not completely suitable for Darwin
platforms:

 * It is a convention to encode the library versioning in the
   binary and to have only one level of symlink for the installed
   files. This needs to be applied to the installation too.
 * The library needs to be built with its correct install name
   so that two-level library naming works.
 * The extension for shared libraries should be .dylib

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
PR jit/100613 - libgccjit should produce dylib on macOS

	PR jit/100613

gcc/jit/ChangeLog:

	* Make-lang.in: Provide clauses for Darwin hosts.