build: Always use EXTERN_UNLESS_MAIN_MODULE pattern.

* common/util.h (EXTERN_UNLESS_MAIN_MODULE): Add the definion only
here but now without the Norcroft-C.  Change all other places where it
gets defined.
* common/iobuf.h (iobuf_debug_mode): Declare unconditionally as
extern.
* common/iobuf.c (iobuf_debug_mode): Define it here.
* agent/gpg-agent.c (INCLUDED_BY_MAIN_MODULE): Define here and also in
all main modules of all other programs.

* g10/main.h: Put util.h before the local header files.
--

This change is required for use with gcc/ld's LTO feature which does
not allow common blocks.  Further gcc 10 will make -fno-common the
default and thus this chnage is always needed.  What a pitty.

Co-authored-by: Tomáš Mráz
GnuPG-bug-id: 4831
Signed-off-by: Werner Koch <wk@gnupg.org>

g13/g13-common.h

@@ -32,7 +32,6 @@
 #include "../common/session-env.h"
 #include "../common/strlist.h"
 
-
 /* Debug values and macros.  */
 #define DBG_MOUNT_VALUE     1	/* Debug mount or device stuff. */
 #define DBG_CRYPTO_VALUE    4	/* Debug low level crypto.  */
@@ -48,6 +47,7 @@
 /* A large struct named "opt" to keep global flags.  Note that this
    struct is used by g13 and g13-syshelp and thus some fields may only
    make sense for one of them.  */
+EXTERN_UNLESS_MAIN_MODULE
 struct
 {
   unsigned int debug; /* Debug flags (DBG_foo_VALUE).  */

g13/g13-syshelp.c

@@ -31,6 +31,7 @@
 #endif
 #include <unistd.h>
 
+#define INCLUDED_BY_MAIN_MODULE 1
 #include "g13-syshelp.h"
 
 #include <gcrypt.h>

g13/g13.c

@@ -27,6 +27,7 @@
 #include <fcntl.h>
 #include <npth.h>
 
+#define INCLUDED_BY_MAIN_MODULE 1
 #include "g13.h"
 
 #include <gcrypt.h>