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.

Dockerfile.arch

@@ -63,12 +63,12 @@ USER root
 RUN pacman -U --noconfirm libstrophe-git/libstrophe-git-*.pkg.tar.zst
 
 WORKDIR /usr/src
-RUN git clone -c http.sslverify=false https://git.jabber.space/devs/stabber
+RUN git clone --depth 1 -c http.sslverify=false https://git.jabber.space/devs/stabber
 
 WORKDIR /usr/src/stabber
 RUN ./bootstrap.sh
 RUN ./configure --prefix=/usr --disable-dependency-tracking
-RUN make
+RUN make -j$(nproc)
 RUN make install
 
 WORKDIR /usr/src/profanity