Generate PDFs in parallel on multi-core systems

Try to speed up the single-threaded PDF generation by starting a process
for each CPU core.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -27,7 +27,8 @@ test-html:
   image: docker-registry.apertis.org/apertis/apertis-v2020-documentation-builder
   stage: generate-pdf
   script:
-  - find public -name "*.pdf-in" -exec scripts/create-pdf {} \;
+  - echo "Building PDFs on $(nproc) cores"
+  - find public -name "*.pdf-in" -print0 | xargs -0 -r -n1 -P "$(nproc)" scripts/create-pdf
   needs:
   - job: test-html
   artifacts: