Add Ja2Export target and get it to build on C++17

nice little tool

ext/export/src/CMakeLists.txt

@@ -0,0 +1,29 @@
+option(BUILD_JA2EXPORT "Build the Ja2Export tool." ON)
+
+if(BUILD_JA2EXPORT)
+	message(STATUS "Configuring Ja2Export")
+
+	add_executable(Ja2Export
+		init_vfs.cpp
+		main.cpp
+		progress_bar.cpp
+		ja2/himage.cpp
+		ja2/XMLWriter.cpp
+		export/jsd/export_jsd.cpp
+		export/jsd/structure.cpp
+		export/slf/export_slf.cpp
+		export/sti/export_sti.cpp
+		export/sti/Image.cpp
+		export/sti/stci_image_utils.cpp
+		export/sti/STCI_lib.cpp
+	)
+	target_include_directories(Ja2Export PUBLIC
+		"${CMAKE_CURRENT_SOURCE_DIR}"
+	)
+	target_link_libraries(Ja2Export PUBLIC bfVFS libpng)
+	set_target_properties(Ja2Export PROPERTIES
+		RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}
+	)
+else()
+	message(STATUS "BUILD_JA2EXPORT set to \"OFF\", not configuring Ja2Export by default")
+endif()