Bump required sdcc version to >= 3.4.0.

Bump required sdcc version to >= 3.4.0.

There is an issue with some sdcc versions that causes problems, e.g. only
the first scan for an FX2 device succeeds, the second one yields
LIBUSB_ERROR_PIPE (see bug #1326).

Recent tests have shown that sdcc 3.3.0 produces broken fx2lafw firmware
files, and sdcc 3.1.0 errors out during the build. Versions 3.4.0, 3.5.0,
3.6.9, and 3.8.0 (for example) all seem to work fine.

Thus, increase the minimum required sdcc version to 3.4.0.

This fixes bug #1326.