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>
(cherry picked from commit 21d9bd8b87)

- Applied respective chnages also to gpg-card and keyboxd.

Signed-off-by: Werner Koch <wk@gnupg.org>

common/util.h

@@ -48,6 +48,14 @@
 #define GPG_ERR_KEYBOXD     317
 #endif /*GPG_ERROR_VERSION_NUMBER*/
 
+#ifndef EXTERN_UNLESS_MAIN_MODULE
+# if !defined (INCLUDED_BY_MAIN_MODULE)
+#  define EXTERN_UNLESS_MAIN_MODULE extern
+# else
+#  define EXTERN_UNLESS_MAIN_MODULE
+# endif
+#endif
+
 /* Hash function used with libksba. */
 #define HASH_FNC ((void (*)(void *, const void*,size_t))gcry_md_write)