TT#48650 Explicitly check for gperf binary

If the gperf dependency isn't present then people
seem to miss that it's not available. Admittedly,
the error message isn't entirely obvious:

| Makefile:135: .depend: No such file or directory
| ../utils/const_str_hash < sdp.c > sdp.strhash.c
| open2: exec of gperf -t -E -l -c -t -I -H __csh_hash -N __csh_lookup_raw failed at ../utils/const_str_hash line 37.

Change-Id: I15edcd0517e88c690be246dc628cb55e25297dd0
Closes: https://github.com/sipwise/rtpengine/issues/718

utils/const_str_hash

@@ -3,6 +3,9 @@
 use strict;
 use warnings;
 use IPC::Open2;
+use IPC::Cmd qw[can_run];
+
+can_run('gperf') or die 'No gperf binary found, make sure to have gperf installed!';
 
 print("/******** GENERATED FILE ********/\n");