Work around linker missing objects when building tests

Since we rely on a global object ctor to add a display output to the
list, it is not referenced from anywhere else, so does not get linked in
when building tests since most objects are pulled from a static library.

Another option would be to use --whole-archive to link to it.

src/display-console.cc

@@ -43,6 +43,7 @@ namespace {
 conky::display_output_console console_output("console");
 
 }  // namespace
+void init_console_output() {}
 
 namespace priv {}  // namespace priv
 

src/display-file.cc

@@ -49,6 +49,7 @@ namespace {
 conky::display_output_file file_output("file");
 
 }  // namespace
+extern void init_file_output() {}
 
 namespace priv {}  // namespace priv
 

src/display-http.cc

@@ -47,6 +47,7 @@ conky::disabled_display_output http_output_disabled("http", "BUILD_HTTP");
 #endif
 
 }  // namespace
+extern void init_http_output() {}
 
 // TODO: cleanup namespace
 // namespace priv {

src/display-ncurses.cc

@@ -52,6 +52,7 @@ conky::disabled_display_output ncurses_output_disabled("ncurses",
 #endif
 
 }  // namespace
+extern void init_ncurses_output() {}
 
 // namespace priv {
 

src/display-output.cc

@@ -50,6 +50,13 @@ display_outputs_t *display_outputs;
 
 }  // namespace
 
+// HACK: force the linker to link all the objects in with test enabled
+extern void init_console_output();
+extern void init_ncurses_output();
+extern void init_file_output();
+extern void init_http_output();
+extern void init_x11_output();
+
 /*
  * The selected and active display output.
  */
@@ -99,6 +106,12 @@ disabled_display_output::disabled_display_output(const std::string &name,
 }
 
 bool initialize_display_outputs() {
+  init_console_output();
+  init_ncurses_output();
+  init_file_output();
+  init_http_output();
+  init_x11_output();
+
   std::vector<display_output_base *> outputs;
   outputs.reserve(display_outputs->size());
 

src/display-x11.cc

@@ -193,6 +193,7 @@ conky::disabled_display_output x11_output_disabled("x11", "BUILD_X11");
 #endif
 
 }  // namespace
+extern void init_x11_output() {}
 
 namespace priv {}  // namespace priv