Support .dylib suffix for modules on macOS (Bug#36226).

Support .dylib suffix for modules on macOS (Bug#36226).

On macOS, shared libraries typically have the suffix .dylib.  This
commit switches the module suffix to .dylib on Darwin to account for
that.  To also support the .so suffix, introduce the concept of a
secondary module suffix.

* configure.ac: Switch MODULES_SUFFIX to .dylib for Darwin, introduce
MODULES_SECONDARY_SUFFIX.

* src/lread.c (Fload, syms_of_lread): Also use
MODULES_SECONDARY_SUFFIX if defined.

* test/src/emacs-module-tests.el (module-darwin-secondary-suffix): New
unit test.