Add gitlab builder configuration to auto-build UG and TRM

.gitignore

 *.log
 *.bak
 *~
+builds/
 **/docs/doxygen/*
 
 # Vivado

.gitlab-ci.yml

+variables:
+  # We just need to check the current and last commit ATM
+  GIT_DEPTH: "2"
+
+stages:
+- build-latex
+
+build-latex:
+  stage: build-latex
+  tags:
+  - latex
+  # FIXME: do it sanely when this is resolved: https://gitlab.com/gitlab-org/gitlab-ce/issues/19232
+  script:
+  - '{ git diff --quiet --exit-code HEAD^ -- docs/technical_reference_manual/ docs/tex_common/ docs/user_guide && printf "No relevant changes in docs/.\n" && exit 1; } || make -C docs/ ug trm'
+  allow_failure: true
+  artifacts:
+    paths:
+    - docs/fiji_trm.pdf
+    - docs/fiji_user_guide.pdf
+    when: on_success

docs/tex_common/header.tex

-% For longer reports use: (starts with chapter) %
-% \documentclass[a4paper,bibtotoc,oneside,parskip=half]{scrbook}
-% For short reports use: (starts with section) %
 \documentclass[a4paper,bibliography=totoc,oneside,parskip=half]{scrartcl}
 
 \usepackage{xr-hyper} % cross-document references, cf. http://www.tex.ac.uk/FAQ-extref.html