[!751] [cmake] Make installation of PDFs from LaTeX optional

Merge branch 'fix/install-latex-doc-optional' into 'master'

ref:core/dune-common Otherwise target "install" fails when target "doc" was
not run.

Fixes [#182]

See merge request [!751]

  [#182]: gitlab.dune-project.org/NoneNone/issues/182
  [!751]: gitlab.dune-project.org/core/dune-common/merge_requests/751


Closes #182

cmake/modules/UseLatexMk.cmake

@@ -239,7 +239,9 @@ function(add_latex_document)
     install(CODE "execute_process(COMMAND ${CMAKE_COMMAND} --build . --target ${LMK_TARGET} --config $<CONFIGURATION>)")
   endif()
   if(LMK_INSTALL)
-    install(FILES ${OUTPUT_PDF} DESTINATION ${LMK_INSTALL})
+    install(FILES ${OUTPUT_PDF}
+            DESTINATION ${LMK_INSTALL}
+            OPTIONAL)
   endif()
 
   # Add a clean up rule to the clean_latex target