check_icmp is missing -D_XPG4_2 on Solaris

[eponymous alias]

The following is being reported against monitoring-plugins-v2.1.1-39-g8095.

In the compilation of plugins-root/check_icmp.c on Solaris 8, I get:

"check_icmp.c", line 948: undefined struct/union member: msg_control
"check_icmp.c", line 948: warning: improper pointer/integer combination: op "="
"check_icmp.c", line 949: undefined struct/union member: msg_controllen
cc: acomp failed for check_icmp.c
*** Error code 2
make: Fatal error: Command failed for target `check_icmp.o'

because struct msghdr is defined in /usr/include/sys/socket.h with
these protections around the affected fields within the structure:

#if defined(_XPG4_2) || defined(_KERNEL)
        void            *msg_control;           /* ancillary data */ 
        socklen_t       msg_controllen;         /* ancillary data buffer len */
        int             msg_flags;              /* flags on received message */
#else
        caddr_t         msg_accrights;  /* access rights sent/received */
        int             msg_accrightslen;
#endif  /* defined(_XPG4_2) || defined(_KERNEL) */

and the check_icmp.c code is referencing the msg_control and
msg_controllen fields.  Clearly, _XPG4_2 needs to be defined automatically
for this compilation, when the Makefile is built in the plugins-root
directory for Solaris.