perf: optimize build with configure cache and parallel make

- Add -C flag to ./configure for caching results
- Use --depth 1 for git clone (faster)
- Use make -j$(nproc) for stabber/libstrophe builds

Borrowed optimizations from build/multicore branch.

ci-build.sh

@@ -107,11 +107,11 @@ esac
 case "$ARCH" in
     linux*)
          echo
-         echo "--> Building with ./configure ${tests[0]} --enable-valgrind $*"
+         echo "--> Building with ./configure -C ${tests[0]} --enable-valgrind $*"
          echo
 
          # shellcheck disable=SC2086
-        ./configure ${tests[0]} --enable-valgrind $*
+        ./configure -C ${tests[0]} --enable-valgrind $*
 
         $MAKE CC="${CC}"
         if grep '^ID=' /etc/os-release | grep -q -e debian; then
@@ -133,13 +133,13 @@ build_and_test() {
 
     {
         echo "=== Build $idx started at $(date) ==="
-        echo "--> Building in $build_dir with ./configure $features $extra_args"
+        echo "--> Building in $build_dir with ./configure -C $features $extra_args"
 
         mkdir -p "$build_dir"
         cd "$build_dir"
 
         # shellcheck disable=SC2086
-        ../configure $features $extra_args
+        ../configure -C $features $extra_args
 
         $MAKE CC="${CC}"
         $MAKE check-functional-parallel