Add build option to disable utmpx update code

On some embedded systems the libc may have utmpx support, but the
feature would be redundant. So add a build switch to disable utmpx
updating, similar to compiling on systems without utmpx support.

Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>

diff -ru vsftpd-3.0.2.orig/builddefs.h vsftpd-3.0.2/builddefs.h
--- vsftpd-3.0.2.orig/builddefs.h	2012-04-05 05:24:56.000000000 +0200
+++ vsftpd-3.0.2/builddefs.h	2014-09-16 14:23:36.128003245 +0200
@@ -4,6 +4,7 @@
 #undef VSF_BUILD_TCPWRAPPERS
 #define VSF_BUILD_PAM
 #undef VSF_BUILD_SSL
+#define VSF_BUILD_UTMPX
 
 #endif /* VSF_BUILDDEFS_H */
 
diff -ru vsftpd-3.0.2.orig/sysdeputil.c vsftpd-3.0.2/sysdeputil.c
--- vsftpd-3.0.2.orig/sysdeputil.c	2012-09-16 06:18:04.000000000 +0200
+++ vsftpd-3.0.2/sysdeputil.c	2014-09-16 14:26:42.686887724 +0200
@@ -1158,7 +1158,7 @@
 
 #endif /* !VSF_SYSDEP_NEED_OLD_FD_PASSING */
 
-#ifndef VSF_SYSDEP_HAVE_UTMPX
+#if !defined(VSF_BUILD_UTMPX) || !defined(VSF_SYSDEP_HAVE_UTMPX)
 
 void
 vsf_insert_uwtmp(const struct mystr* p_user_str,
@@ -1173,7 +1173,7 @@
 {
 }
 
-#else /* !VSF_SYSDEP_HAVE_UTMPX */
+#else /* !VSF_BUILD_UTMPX || !VSF_SYSDEP_HAVE_UTMPX */
 
 /* IMHO, the pam_unix module REALLY should be doing this in its SM component */
 /* Statics */
@@ -1238,7 +1238,7 @@
   updwtmpx(WTMPX_FILE, &s_utent);
 }
 
-#endif /* !VSF_SYSDEP_HAVE_UTMPX */
+#endif /* !VSF_BUILD_UTMPX || !VSF_SYSDEP_HAVE_UTMPX */
 
 void
 vsf_set_die_if_parent_dies()