Makefile: auto-target automatically pass --program-prefix=""

When cross-compiled, some packages incorrectly prefix their executables
with the target tuple.

Fix that by forcing the program prefix to the empty string when calling
./configure, but only when compiling for target, not for host.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>