Fixed|GCC: Conflicts between C/C++ linkage of extern variables

As we switch souce files to C++, we must be careful of the implications
to externed global variables. They must be declared to use C linkage
until all source files are fully C++ (if we ever reach that point).

doomsday/plugins/jdoom64/include/r_defs.h

@@ -114,11 +114,11 @@ typedef struct xline_s {
 } xline_t;
 
 // Our private map data structures.
-extern xsector_t* xsectors;
-extern xline_t* xlines;
+DENG_EXTERN_C xsector_t* xsectors;
+DENG_EXTERN_C xline_t* xlines;
 
 // If true we are in the process of setting up a map.
-extern boolean mapSetup;
+DENG_EXTERN_C boolean mapSetup;
 
 /**
  * Converts a line to an xline.

doomsday/plugins/jheretic/include/doomdef.h

@@ -54,8 +54,8 @@
 #define VERBOSE(code)       { if(verbose >= 1) { code; } }
 #define VERBOSE2(code)      { if(verbose >= 2) { code; } }
 
-extern game_import_t gi;
-extern game_export_t gx;
+DENG_EXTERN_C game_import_t gi;
+DENG_EXTERN_C game_export_t gx;
 
 //
 // Global parameters/defines.

doomsday/plugins/jhexen/include/h2def.h

@@ -85,8 +85,8 @@
 #define VERBOSE(code)       { if(verbose >= 1) { code; } }
 #define VERBOSE2(code)      { if(verbose >= 2) { code; } }
 
-extern game_import_t gi;
-extern game_export_t gx;
+DENG_EXTERN_C game_import_t gi;
+DENG_EXTERN_C game_export_t gx;
 
 //
 // Global parameters/defines.

doomsday/plugins/jhexen/include/x_state.h

@@ -51,14 +51,14 @@ extern boolean  paused;            // Game Pause?
 
 // This one is related to the 3-screen display mode.
 // ANG90 = left side, ANG270 = right
-extern int      viewangleoffset;
+DENG_EXTERN_C int viewangleoffset;
 
 // Timer, for scores.
-extern int mapStartTic; // Game tic at map start.
-extern int mapTime; // Tics in game play for par.
+DENG_EXTERN_C int mapStartTic; // Game tic at map start.
+DENG_EXTERN_C int mapTime; // Tics in game play for par.
 
 // Quit after playing a demo from cmdline.
-extern boolean singledemo;
+DENG_EXTERN_C boolean singledemo;
 
 //-----------------------------
 // Internal parameters, fixed.