MDEV-20685: compile fixes for Solaris/OSX/AIX

sig_return: Solaris/OSX returns different function ptr
Move defination to my_alarm.h as its the only use.

prevents compile warnings (copied from 10.3 branch)

mysys/my_sync.c:136:19: error: 'cur_dir_name' defined but not used [-Werror=unused-const-variable=]
  136 | static const char cur_dir_name[]= {FN_CURLIB, 0};
      |                   ^~~~~~~~~~~~

fix compile error (DEPRECATED) leaked from ssl headers.

In file included from /export/home/dan/mariadb-server-10.4/sql/sys_vars.cc:37:
/export/home/dan/mariadb-server-10.4/sql/sys_vars.ic:69: error: "DEPRECATED" redefined [-Werror]
   69 | #define DEPRECATED(X) X
      |
In file included from /export/home/dan/mariadb-server-10.4/include/violite.h:150,
                 from /export/home/dan/mariadb-server-10.4/sql/sql_class.h:38,
                 from /export/home/dan/mariadb-server-10.4/sql/sys_vars.cc:36:
/usr/include/openssl/ssl.h:2356: note: this is the location of the previous definition
 2356 | # define DEPRECATED __attribute__((deprecated))
      |

Avoid Werror condition on non-Linux:

plugin/server_audit/server_audit.c:2267:7: error: variable 'db_len_off' set but not used [-Werror=unused-but-set-variable]
 2267 |   int db_len_off;
      |       ^~~~~~~~~~
plugin/server_audit/server_audit.c:2266:7: error: variable 'db_off' set but not used [-Werror=unused-but-set-variable]
 2266 |   int db_off;
      |       ^~~~~~

auth_gssapi fix include path for Solaris

Consistent with the upstream packaged patch:
https://github.com/OpenIndiana/oi-userland/blob/oi/hipster/components/database/mariadb-103/patches/06-gssapi.h.patch

compile warnings on Solaris

[ 91%] Building C object plugin/server_audit/CMakeFiles/server_audit.dir/server_audit.c.o
/plugin/server_audit/server_audit.c: In function 'auditing_v8':
/plugin/server_audit/server_audit.c:2194:20: error: unused variable 'db_len_off' [-Werror=unused-variable]
 2194 |   static const int db_len_off= 128;
      |                    ^~~~~~~~~~
/plugin/server_audit/server_audit.c:2193:20: error: unused variable 'db_off' [-Werror=unused-variable]
 2193 |   static const int db_off= 120;
      |                    ^~~~~~
/plugin/server_audit/server_audit.c:2192:20: error: unused variable 'cmd_off' [-Werror=unused-variable]
 2192 |   static const int cmd_off= 4432;
      |                    ^~~~~~~
At top level:
/plugin/server_audit/server_audit.c:2192:20: error: 'cmd_off' defined but not used [-Werror=unused-const-variable=]
/plugin/server_audit/server_audit.c:2193:20: error: 'db_off' defined but not used [-Werror=unused-const-variable=]
 2193 |   static const int db_off= 120;
      |                    ^~~~~~
/plugin/server_audit/server_audit.c:2194:20: error: 'db_len_off' defined but not used [-Werror=unused-const-variable=]
 2194 |   static const int db_len_off= 128;
      |                    ^~~~~~~~~~
cc1: all warnings being treated as errors

tested on:
$ uname -a
SunOS openindiana 5.11 illumos-b97b1727bc i86pc i386 i86pc

1 files changed, 1 insertions, 2 deletions

diff --git a/mysys/my_sync.c b/mysys/my_sync.c
index 2ed62e4541a..5a21a511dda 100644
--- a/mysys/my_sync.c
+++ b/mysys/my_sync.c
@@ -133,8 +133,6 @@ int my_sync(File fd, myf my_flags)
 } /* my_sync */
 
 
-static const char cur_dir_name[]= {FN_CURLIB, 0};
-
 /*
   Force directory information to disk.
 
@@ -151,6 +149,7 @@ int my_sync_dir(const char *dir_name __attribute__((unused)),
                 myf my_flags __attribute__((unused)))
 {
 #ifdef NEED_EXPLICIT_SYNC_DIR
+  static const char cur_dir_name[]= {FN_CURLIB, 0};
   File dir_fd;
   int res= 0;
   const char *correct_dir_name;