maint: remove GLOBAL as per GCC 14

* src/common.h (GLOBAL): Remove this macro, and all its uses.
It collides with GCC 14 and -Wmissing-variable-declarations.
Change all uses of GLOBAL to use extern instead,
and declare the variables in their respective .c files.
Move .c file’s extern declarations here, so that they
appear only once and are checked against definitions.
* src/names.c (unconsumed_option_tail): Now static.

src/extract.c

@@ -30,6 +30,8 @@
 
 #include "common.h"
 
+dev_t root_device;
+
 static bool we_are_root;	/* true if our effective uid == 0 */
 static mode_t newdir_umask;	/* umask when creating new directories */
 static mode_t current_umask;	/* current umask (which is set to 0 if -p) */