- always use `-Wextra` compilation flag; it was already used in CI
- always compile `ndpiSimpleIntegration` when building examples
- don't mess with optimization flags: `CFLAGS` default value is "-g -O2"
and the user can change it
Try to test -O1,2,3,s flags in CI.
Fix some warnings.
Initial work to support out-of-tree builds
```
./autogen.sh
mkdir build
cd build
../configure
make
make check
```
IMPORTANT: `autogen.sh` doesn't call `configure` automatically anymore!!
You have to do: `./autogen.sh && ./configure --$OPTIONS`.
A little bit annoying but the pattern `autogen && configure && make` is
very common on Linux.
Known issues:
* `make doc` doesn't work in out-of-tree builds, yet
* Windows/MinGW/DPDK (out-of-tree) builds have not been tested, so it is unlikely they work
See: #2992
