Support SOURCE_DATE_EPOCH

Support SOURCE_DATE_EPOCH

If set, it is used to set the "created" and "modified" fields in the
OpenType 'head' table.  This means that builds of Bedstead can be
reproducible.

The current code just casts the "long long" interpretation of the
environment variable into a time_t.  This is potentially undefined
behaviour, because time_t might be a signed integer type smaller than
"long long".  But I can't find a way to properly range-check it.  Even
in POSIX, where time_t is required to be an integer type, there
doesn't seem to be a constant that specifies its range.