Have some C++ file in the project

Rename gtk-gui.c to qt-gui.cpp, and make the necessary changes so that
the project still builds.

Signed-off-by: Alberto Mardegan <mardy@users.sourceforge.net>

1 files changed, 8 insertions, 0 deletions

diff --git a/display.h b/display.h
index 8200770d7..d5c69e81b 100644
--- a/display.h
+++ b/display.h
@@ -3,6 +3,10 @@
 
 #include <cairo.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define SCALE_SCREEN 1.0
 #define SCALE_PRINT (1.0 / get_screen_dpi())
 
@@ -63,4 +67,8 @@ struct options {
 
 extern char zoomed_plot, dc_number;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif