compile help in headless mode

Author: MjnMixael

qtfred/CMakeLists.txt

@@ -93,7 +93,8 @@ file(GLOB_RECURSE QTFRED_HELP_SOURCES
 
 add_custom_command(
     OUTPUT "${QTFRED_HELP_QCH}"
-    COMMAND "${QHELPGENERATOR_EXECUTABLE}" "${QTFRED_HELP_QHP}" -o "${QTFRED_HELP_QCH}"
+    COMMAND ${CMAKE_COMMAND} -E env "QT_QPA_PLATFORM=offscreen"
+        "${QHELPGENERATOR_EXECUTABLE}" "${QTFRED_HELP_QHP}" -o "${QTFRED_HELP_QCH}"
     DEPENDS ${QTFRED_HELP_SOURCES}
     COMMENT "Compiling QtFRED help (qtfred_help.qch)"
     VERBATIM)