I added the test if an optional parameter is not empty, but that doesn't work
with -u. Provide an empty "fallback" value to fix the issue.
Also group the update steps so that it's easier to see what is going on.
Shell scripts should be executable so that meson reports their
invocation succinctly (does not print 'sh' '-e').
Python scripts should not be executable so that meson does the
detection of the right python binary itself.
Add -u everywhere to catch potential errors.
libcurl is already our build dependency, so using curl reduces the deps
a bit, and curl also has a more modern codebase.
Regenerating the patch makes it more likely that the patch will apply
in the future.
Also, update URLs which return 302 to the new location.
(Patch suggested by Igor Gnatenko.)
