Avoid implicit declaration of the exit function, to increase
compatibility with future compilers which are likely to reject
implicit function declarations by default.

Submitted upstream: <https://github.com/Cacti/spine/pull/290>

diff --git a/configure.ac b/configure.ac
index d2d0303684fd36dc..7ee8c427dff87286 100644
--- a/configure.ac
+++ b/configure.ac
@@ -359,7 +359,7 @@ fi
 
 # Net-SNMP includes v3 support and insists on crypto unless compiled --without-openssl
 AC_MSG_CHECKING([if Net-SNMP needs crypto support])
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <net-snmp-config.h>]], [[exit(NETSNMP_USE_OPENSSL != 1);]])],[  AC_MSG_RESULT(yes)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <net-snmp-config.h>]], [[return NETSNMP_USE_OPENSSL != 1;]])],[  AC_MSG_RESULT(yes)
      SNMP_SSL=yes
   ],[AC_MSG_RESULT(no)
 ])