MFH: r518294

Prevent failure deep in the build if dtrace is not installed:

dtrace_cython/consumer.c:611:10: fatal error: 'sys/dtrace.h' file not found

Present a more informative message instead.

This affects (only some?) of the package building machines, e.g., for
powerpc64.

Approved by: portmgr (tier-2 blanket)