Add the -C flag to ./configure to enable caching, reducing redundant
resource fetching during builds when packages and the machine remain
unchanged. Caching may cause issues only if applied across different machines
or if packages are modified, neither of which is the case during the build.
`--depth 1` flag ensures that only latest version is loaded (since there is no need to sync full git)
Using local libstrophe version increases speed of fetching
nproc is a standard util that prints amount of available processes.
-j is a flag that specifies how many cores can be utilized by `make` (default is 1)
-j$(nproc) allows to use all the available machine cores, potentially significantly speeding up completion of CI actions
* Also pass `$*` to `configure` when invoking `ci-build.sh`, so one can
e.g. run `./ci-build.sh --without-xscreensaver`
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
