Restore compatibility with CMake < 3.12

CMakeLists.txt

@@ -378,6 +378,12 @@ if(CMAKE_COMPILER_IS_GNUCXX)
 
     set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O3 -g -DNDEBUG")
     set(CMAKE_C_FLAGS_RELWITHDEBINFO  "-O3 -g -DNDEBUG")
+
+    if(HAVE_QT)
+        # if Qt is built with reduce-relocations, applicatino code 
+        # needs to be compiled with -fPIC to match
+        set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+    endif()
 endif(CMAKE_COMPILER_IS_GNUCXX)
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" OR CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang" )
@@ -482,15 +488,15 @@ include(SetupFGFSLibraries)
 # create an object library target to hold all our sources
 # subdirectories will append sources to this target, and we'll
 # link it into our real executable targets
+# use of 'flightgear-version' text file here is a dummy; CMake 3.10
+# requries at least one direct source file for targets, even though
+# we use target_sources() later
+
+add_library(fgfsObjects OBJECT flightgear-version)
 
-add_library(fgfsObjects OBJECT)
-# Set up the include search paths for the object library
-setup_fgfs_libraries(fgfsObjects)
-setup_fgfs_includes(fgfsObjects)
 
 ########################################################################
 
-
 add_subdirectory(3rdparty)
 add_subdirectory(utils)
 
@@ -500,6 +506,11 @@ add_subdirectory(man)
 add_subdirectory(package)
 add_subdirectory(scripts)
 
+# Set up the include search paths for the object library : has to be done
+# after targets are fully defined
+setup_fgfs_library_includes(fgfsObjects)
+setup_fgfs_includes(fgfsObjects)
+
 #----------------------------------------------------------------------------
 ### MSVC startup project - ensure you can just hit build & run in MSVC
 set_property(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY VS_STARTUP_PROJECT fgfs)

CMakeModules/SetupFGFSLibraries.cmake

@@ -67,3 +67,78 @@ function(setup_fgfs_libraries target)
         target_link_libraries(${target} sentry::sentry)
     endif()
 endfunction()
+
+
+# CMake < 3.12 can't define a link to an OBJECT library to specify its include
+# paths, so we have to essentially duplicate the above and configure the paths manually.
+# Once we require CMake 3.12, delete all this and use the function above.
+
+function (_apply_target_includes dest target)
+    if (NOT TARGET ${target})
+        return()
+    endif()
+
+    # retrieve the list of includes from the target
+    get_target_property(includes ${target} INTERFACE_INCLUDE_DIRECTORIES)
+
+    foreach(i ${includes})
+        # skip any invalid includes
+        if (NOT i)
+            return()
+        endif()
+
+        target_include_directories(${dest} PUBLIC ${i})
+    endforeach()
+endfunction()
+
+function (_apply_all_target_includes dest)
+    foreach(arg IN LISTS ARGN)
+      _apply_target_includes(${dest} ${arg})
+    endforeach()
+endfunction()
+
+function (setup_fgfs_library_includes target)
+ 
+    _apply_all_target_includes(${target} 
+        SimGearCore
+        SimGearScene
+        Boost::boost
+        ${EVENT_INPUT_LIBRARIES}
+        ${HLA_LIBRARIES}
+        ${OPENGL_LIBRARIES}
+        ${OPENSCENEGRAPH_LIBRARIES}
+        ${PLATFORM_LIBS}
+        ${PLIB_LIBRARIES}
+    )
+
+    if(ENABLE_JSBSIM)
+        _apply_target_includes(${target} JSBSim)
+    endif()    
+    
+    _apply_target_includes(${target} fgsqlite3)
+    _apply_target_includes(${target} fgvoicesynth)
+    _apply_target_includes(${target} fgembeddedresources)
+
+    if (HAVE_QT)
+        _apply_all_target_includes(${target} Qt5::Core Qt5::Widgets Qt5::Gui)
+    endif()
+
+    if(ENABLE_IAX)
+        _apply_target_includes(${target} iaxclient_lib)
+    endif()
+
+    if (ENABLE_SWIFT)
+        _apply_all_target_includes(${target} ${dbus_target})
+    endif()
+
+    if (ENABLE_PLIB_JOYSTICK)
+        _apply_target_includes(${target} PLIBJoystick)
+    endif()
+    
+    _apply_target_includes(${target} PLIBFont)
+
+    if (TARGET sentry::sentry)
+        _apply_target_includes(${target} sentry::sentry)
+    endif()
+
+endfunction()