diff -rupN gmsh-2.12.0-source/Fltk/FlGui.cpp gmsh-2.12.0-source-new/Fltk/FlGui.cpp
--- gmsh-2.12.0-source/Fltk/FlGui.cpp	2016-03-05 12:58:58.000000000 +0100
+++ gmsh-2.12.0-source-new/Fltk/FlGui.cpp	2016-03-06 11:51:12.461853229 +0100
@@ -370,7 +370,7 @@ FlGui::FlGui(int argc, char **argv)
   // nothing to do here
 #else
   fl_open_display();
-  static char gmsh32x32[] = {
+  static unsigned char gmsh32x32[] = {
     0x00, 0x00, 0x00, 0x00, 0x00, 0x80, 0x01, 0x00, 0x00, 0x40, 0x03, 0x00,
     0x00, 0x40, 0x03, 0x00, 0x00, 0x20, 0x07, 0x00, 0x00, 0x20, 0x07, 0x00,
     0x00, 0x10, 0x0f, 0x00, 0x00, 0x10, 0x0f, 0x00, 0x00, 0x08, 0x1f, 0x00,
@@ -384,7 +384,7 @@ FlGui::FlGui(int argc, char **argv)
     0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0x00};
   graph[0]->getWindow()->icon
     ((const char*)XCreateBitmapFromData(fl_display, DefaultRootWindow(fl_display),
-                                        gmsh32x32, 32, 32));
+                                        reinterpret_cast<char*>(gmsh32x32), 32, 32));
 #endif
 
   graph[0]->getWindow()->show(argc >0 ? 1 : 0, argv);