Add BSDmakefile to prevent errors when `make` is called under FreeBSD (#4446)

The syntax of the gitea Makefile is not platform-agnostic and is
specific to the GNU version of `make`. BSD platforms such as FreeBSD
ship with bmake (BSD make) as their default `make` program; attempting
to compile gitea by simply executing `make` causes a wall of errors to
show as a result of syntax incompatible with BSD make.

If a file named `BSDmakefile` is present, `bmake` will give it
preference over a generic `Makefile`. This `BSDmakefile` is taken from
the BSD-licensed `gmake-proxy` project [0], which transparently proxies
all `make` commands to `gmake` (GNU make) on systems where `bmake` is
the default, and if `gmake` is not installed an error message is
displayed.

[0]: https://github.com/neosmart/gmake-proxy

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