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.debian

@@ -39,19 +39,19 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 RUN mkdir -p /usr/src/{stabber,libstrophe,profanity}
 WORKDIR /usr/src
 
-RUN git clone -c http.sslverify=false https://git.jabber.space/devs/stabber
-RUN git clone -c http.sslverify=false https://github.com/strophe/libstrophe
+RUN git clone --depth 1 -c http.sslverify=false https://git.jabber.space/devs/stabber
+RUN git clone --depth 1 -c http.sslverify=false https://github.com/strophe/libstrophe
 
 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/libstrophe
 RUN ./bootstrap.sh
 RUN ./configure --prefix=/usr
-RUN make
+RUN make -j$(nproc)
 RUN make install
 
 WORKDIR /usr/src/profanity