Enable FFI modules to be used as statically linked plugins.

If the generated C code for a module is compiled with -DSTATIC_LINKAGE, the initModule() function is renamed to something based on the module name, e.g. initFMAlloc() for Foreign.Marshal.Alloc (abbreviated for limited linkers).

IIRC, when I wrote the static linkage code, using this required you to either modify plugins.c or compile plugins.c with -DPLUGINS='{"Foreign.Marshal.Alloc", initFMAlloc}' These work but could obviously get a bit tedious/ hard to maintain. If you haven't already done so, it would be very, very cool to make use of the ELF / DLL initializer feature. My thought is that you could replace the 'pluginList' array in plugins.c with a linked list (changing the 'havePlugin' code as required) and that initializers would just add themselves to the linked list. (Obviously, you'd want to retain a non-ELF/DLL option as well.) It's tempting to make the initFMAlloc function an ELF initializer directly but that won't work because Hugs isn't initialized and because the code that implements foreign export is supposed to be able to perform a lookup when the module is initialized. (I say 'supposed' because the frontend parts of foreign export still don't exist.) -- Alastair