Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

Use the high order bits for the intrinsic value type (hint, integer, float
or string) and the low order ones for the more specific sub-types.

Values are 8-bits.

Variable types and configuration item types are separate. Fortunately
they use the same bits so it worked fine.

If the high nibble is set, the type is item type and the low nibble is
necessarily zero.

The FFmpeg configure script autodetects those correctly. Of course,
this assumes that the compiler defaults or flags are adequate.

When the shell evaluates 'foo | bar', it returns the status of bar, not
foo. 'foo' can fail silently: so long as 'bar' succeeds, make will
not notice and will therefore not delete the target.

So lets use 'tar cv(z|j|J)' instead of 'tar cv | (gzip|bzip2|xz)'.

Might close #4907 and #4904 (tbc)

wget uses the timestamp from the server, which is nice in theory, but
breaks makefile dependency propagation.