Build system: minor fixes about flag compilation and example dependencies (#3038)

- 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.

.github/workflows/build_scheduled.yml

@@ -112,7 +112,7 @@ jobs:
     name: Thread Sanitizer (ubuntu-latest)
     runs-on: ubuntu-latest
     env:
-      CFLAGS: -Wextra -Werror
+      CFLAGS: -Werror
     steps:
       - uses: actions/checkout@v4
         with:
@@ -137,7 +137,7 @@ jobs:
     name: Local gcrypt on ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
     env:
-      CFLAGS: -Wextra -Werror -DNDPI_EXTENDED_SANITY_CHECKS
+      CFLAGS: -Werror -DNDPI_EXTENDED_SANITY_CHECKS
     strategy:
       fail-fast: true
       matrix:
@@ -175,7 +175,7 @@ jobs:
     name: Local gcrypt on ${{ matrix.os }} (msys2)
     runs-on: ${{ matrix.os }}
     env:
-      CFLAGS: -Wextra -Werror -DNDPI_EXTENDED_SANITY_CHECKS
+      CFLAGS: -Werror -DNDPI_EXTENDED_SANITY_CHECKS
     strategy:
       fail-fast: true
       matrix: