Add support for out-of-tree builds (#2993)

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

.github/workflows/codeql.yml

@@ -40,7 +40,7 @@ jobs:
           pprof -h
       - name: Configure nDPI library
         run: |
-          ./autogen.sh --enable-gprof --enable-option-checking=fatal --with-pcre2 --with-maxminddb --enable-tls-sigs --enable-debug-messages
+          ./autogen.sh && ./configure --enable-gprof --enable-option-checking=fatal --with-pcre2 --with-maxminddb --enable-tls-sigs --enable-debug-messages
 
       - name: Initialize CodeQL
         uses: github/codeql-action/init@v3