Smarter BSDmakefile that automatically calls gmake to build the targets,
even including `-j` if provided. README.md can be simplified to remove
`gmake` references from build instructions for BSD users.
On BSD platforms, a BSD-specific BSDmakefile is searched for and used
before any generic Makefile. We can use this to emit an informational
message directing the user to use GNU Make instead of relying on the
user's recognizing of random build failures on syntax errors as a sign
to switch to GNU Make.
(Random fact: this same trick also applies to GNU Make, which searches
for a GNUmakefile before using Makefile)
