Remove empty generated file upon gperf failure

Prior to this change the build could fail as follows, if gperf is not
available (or fails):

• make(1) stops due to the gperf error, but an empty target file (cfns.h) is still created
• the empty cfns.h is newer than the source cfns.gperf so it is not regenerated on subsequent builds
• the gcc build fails (undefined reference to libc_name_p)

Sponsored by: The FreeBSD Foundation
MFC after: 3 days