ci: Select number of parallel make jobs depending on CI environment

This should improve compilation times on macOS. Things can certainly
be improved further, e.g., by running the benchmarks in parallel.

.cirrus.yml

@@ -16,6 +16,7 @@ env:
   CTIMETEST: yes
   BENCH: yes
   ITERS: 2
+  MAKEFLAGS: -j2
 
 cat_logs_snippet: &CAT_LOGS
   always:
@@ -131,6 +132,8 @@ task:
   env:
     HOMEBREW_NO_AUTO_UPDATE: 1
     HOMEBREW_NO_INSTALL_CLEANUP: 1
+    # Cirrus gives us a fixed number of 12 virtual CPUs. Not that we even have that many jobs at the moment...
+    MAKEFLAGS: -j13
   matrix:
     << : *ENV_MATRIX
   matrix:

ci/cirrus.sh

@@ -21,7 +21,8 @@ valgrind --version || true
     --with-valgrind="$WITH_VALGRIND" \
     --host="$HOST" $EXTRAFLAGS
 
-make -j2
+# We have set "-j<n>" in MAKEFLAGS.
+make
 
 # Print information about binaries so that we can see that the architecture is correct
 file *tests || true
@@ -30,7 +31,7 @@ file .libs/* || true
 
 if [ -n "$BUILD" ]
 then
-    make -j2 "$BUILD"
+    make "$BUILD"
 fi
 
 if [ "$RUN_VALGRIND" = "yes" ]