doc: Set -dNOSAFER environment for ghostscript

This fixes issues when using LaTeX in doxygen

cmake/enable_doxygen_documentation.cmake

@@ -43,7 +43,7 @@ if(DOXYGEN_FOUND)
   configure_file(${CMAKE_CURRENT_SOURCE_DIR}/cmake/Doxyfile.in ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile @ONLY)
 
   add_custom_target(doc ${DOC_DEPENDENCY}
-    ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
+    COMMAND ${CMAKE_COMMAND} -E env GS_OPTIONS=-dNOSAFER ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
     )
   #note the / after ${CMAKE_BINARY_DIR}/doc/. This causes the directory to be renamed to the destination, not copied into