cmake: add option HELLOIMGUI_DOWNLOAD_THIRD (OFF under Linux)

CMakeLists.txt

@@ -51,6 +51,19 @@ if(MACOSX)
     option(HELLOIMGUI_MACOS_NO_BUNDLE "Under macOS, build regular terminal executables, not app bundles" OFF)
 endif()
 
+#------------------------------------------------------------------------------
+# HELLOIMGUI_DOWNLOAD_THIRD: download third party libraries as part of the build
+#     This is the default, except under Linux, where it is expected that the libraries are installed system-wide
+#     When ON, the libraries that can be downloaded are SDL and Glfw. This is governed by the options
+#     HELLOIMGUI_WITH_GLFW and HELLOIMGUI_WITH_SDL, where HELLOIMGUI_WITH_GLFW is set to ON if no other
+#     options is selected (see below)
+#------------------------------------------------------------------------------
+if (CMAKE_SYSTEM_NAME MATCHES "Linux")
+    option(HELLOIMGUI_DOWNLOAD_THIRD "Automatically download third party libraries during the build" OFF)
+else()
+    option(HELLOIMGUI_DOWNLOAD_THIRD "Automatically download third party libraries during the build" ON)
+endif()
+
 #------------------------------------------------------------------------------
 # Options / Backend selection
 #------------------------------------------------------------------------------
@@ -61,6 +74,7 @@ if (NOT HELLOIMGUI_WITH_GLFW
     AND NOT HELLOIMGUI_USE_GLFW_OPENGL3
     AND NOT HELLOIMGUI_USE_QT
     AND NOT EMSCRIPTEN
+    AND HELLOIMGUI_DOWNLOAD_THIRD
     )
     set(HELLOIMGUI_WITH_GLFW ON CACHE BOOL "")
     message(WARNING "