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)