Ticket #3428: fix -Wredundant-decls compiler warnings...

...and use compiler option for future compile checks.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

lib/filehighlight/ini-file-read.c

@@ -38,8 +38,6 @@
 
 /*** global variables ****************************************************************************/
 
-extern mc_skin_t mc_skin__default;
-
 /*** file scope macro definitions ****************************************************************/
 
 /*** file scope type declarations ****************************************************************/

lib/skin/internal.h

@@ -20,8 +20,6 @@ typedef struct mc_skin_color_struct
 
 /*** global variables defined in .c file *********************************************************/
 
-extern mc_skin_t mc_skin__default;
-
 /*** declarations of public functions ************************************************************/
 
 gboolean mc_skin_ini_file_load (mc_skin_t *);

lib/tty/tty-slang.c

@@ -57,8 +57,6 @@
 
 /*** global variables ****************************************************************************/
 
-extern int reset_hp_softkeys;
-
 /*** file scope macro definitions ****************************************************************/
 
 #ifndef SLTT_MAX_SCREEN_COLS

m4.include/mc-cflags.m4

@@ -63,6 +63,7 @@ dnl    MC_CHECK_ONE_CFLAG([-Wbad-function-cast])
     MC_CHECK_ONE_CFLAG([-Wno-unreachable-code])
     MC_CHECK_ONE_CFLAG([-Wparentheses])
     MC_CHECK_ONE_CFLAG([-Wpointer-sign])
+    MC_CHECK_ONE_CFLAG([-Wredundant-decls])
     MC_CHECK_ONE_CFLAG([-Wreturn-type])
     MC_CHECK_ONE_CFLAG([-Wsequence-point])
     MC_CHECK_ONE_CFLAG([-Wshadow])