Merge branch '109-ci-multithreading' into 'develop'

Resolve "CI multithreading"

Closes #109

See merge request !92

.gitlab-ci.yml

@@ -23,7 +23,7 @@ build_ubuntu_gcc:
     - mkdir debug
     - cd debug
     - cmake -DCMAKE_BUILD_TYPE=Debug ..
-    - cmake --build .
+    - cmake --build . -j 4
   artifacts:
     name: "Documentation - $CI_PIPELINE_ID"
     paths:
@@ -49,7 +49,7 @@ build_win10_msvc:
     - mkdir debug
     - cd debug
     - cmake -DCMAKE_BUILD_TYPE=Debug ..
-    - cmake --build .
+    - cmake --build . -j 4
 
 build_win10_mingw:
   only:
@@ -85,7 +85,7 @@ build_mac_clang:
     - export LDFLAGS="-L/usr/local/opt/llvm/lib"
     - export CPPFLAGS="-I/usr/local/opt/llvm/include"
     - cmake -DCMAKE_C_COMPILER="/usr/local/opt/llvm/bin/clang" -DCMAKE_CXX_COMPILER="/usr/local/opt/llvm/bin/clang++" -DCMAKE_BUILD_TYPE=Debug ..
-    - cmake --build .
+    - cmake --build . -j 4
 
 deploy_doc_develop:
   only: