Avoiding:

cc1: warnings being treated as errors
datum.c: In function `DatumGetSize':
datum.c:57: warning: unsigned value >= 0 is always 1
gmake[3]: *** [datum.o] Error 1

There was:
    if (byVal) {
        if (len >= 0 && len <= sizeof(Datum)) {

but len has type Size (unsigned int) and so now there is:
    if (byVal) {
        if (len <= sizeof(Datum)) {