Fix the "Custom skincolors" branch not compiling with GCC 10.

src/d_main.c

@@ -125,6 +125,9 @@ boolean advancedemo;
 INT32 debugload = 0;
 #endif
 
+UINT16 numskincolors;
+menucolor_t *menucolorhead, *menucolortail;
+
 char savegamename[256];
 
 char srb2home[256] = ".";
@@ -1190,7 +1193,7 @@ void D_SRB2Main(void)
 
 	if (M_CheckParm("-password") && M_IsNextParm())
 		D_SetPassword(M_GetNextParm());
-	
+
 	// player setup menu colors must be initialized before
 	// any wad file is added, as they may contain colors themselves
 	M_InitPlayerSetupColors();

src/doomdef.h

@@ -396,7 +396,7 @@ typedef enum
 	NUMSUPERCOLORS = ((SKINCOLOR_FIRSTFREESLOT - FIRSTSUPERCOLOR)/5)
 } skincolornum_t;
 
-UINT16 numskincolors;
+extern UINT16 numskincolors;
 
 extern skincolor_t skincolors[MAXSKINCOLORS];
 

src/m_menu.h

@@ -450,7 +450,7 @@ typedef struct menucolor_s {
 	UINT16 color;
 } menucolor_t;
 
-menucolor_t *menucolorhead, *menucolortail;
+extern menucolor_t *menucolorhead, *menucolortail;
 
 void M_AddMenuColor(UINT16 color);
 void M_MoveColorBefore(UINT16 color, UINT16 targ);