Use a stronger X font definition for X11_MessageBox on UTF-8

X11 seemed to be confused by the broad definition, so WEIGHT_NAME,
SLANT and SETWIDTH_NAME were defined, thus fixing the font lookup
on some systems (tested on Mageia 6 with X11 1.19.1).

Fixes bug 3571.

src/video/x11/SDL_x11messagebox.c

@@ -50,7 +50,7 @@
 #define MIN_DIALOG_HEIGHT       100     /* Minimum dialog height */
 
 static const char g_MessageBoxFontLatin1[] = "-*-*-medium-r-normal--0-120-*-*-p-0-iso8859-1";
-static const char g_MessageBoxFont[] = "-*-*-*-*-*-*-*-120-*-*-*-*-*-*";
+static const char g_MessageBoxFont[] = "-*-*-medium-r-normal--*-120-*-*-*-*-*-*";
 
 static const SDL_MessageBoxColor g_default_colors[ SDL_MESSAGEBOX_COLOR_MAX ] = {
     { 56,  54,  53  }, /* SDL_MESSAGEBOX_COLOR_BACKGROUND, */