Ignore .dll files

Signed-off-by: Claudio Bley <cbley@av-test.de>
Fix generation of headers when cross compiling

Use the EXEEXT_FOR_BUILD when calling the generated desdata
program, otherwise the build fails when cross compiling because
keymap.h and rotors.h are not found.

Signed-off-by: Claudio Bley <cbley@av-test.de>
ChangeLog for EXEEXT_FOR_BUILD fix.