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

BSDmakefile

@@ -0,0 +1,51 @@
+# GNU makefile proxy script for BSD make
+# Written and maintained by Mahmoud Al-Qudsi <mqudsi@neosmart.net>
+# Copyright NeoSmart Technologies <https://neosmart.net/> 2014-2018
+# Obtain updates from <https://github.com/neosmart/gmake-proxy>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+# 1. Redistributions of source code must retain the above copyright notice, this
+# list of conditions and the following disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright notice,
+# this list of conditions and the following disclaimer in the documentation
+# and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+JARG =
+GMAKE = "gmake"
+#When gmake is called from another make instance, -w is automatically added
+#which causes extraneous messages about directory changes to be emitted.
+#--no-print-directory silences these messages.
+GARGS = "--no-print-directory"
+
+.if "$(.MAKE.JOBS)" != ""
+JARG = -j$(.MAKE.JOBS)
+.endif
+
+#by default bmake will cd into ./obj first
+.OBJDIR: ./
+
+.PHONY: FRC
+$(.TARGETS): FRC
+	$(GMAKE) $(GARGS) $(.TARGETS:S,.DONE,,) $(JARG)
+
+.DONE .DEFAULT: .SILENT
+	$(GMAKE) $(GARGS) $(.TARGETS:S,.DONE,,) $(JARG)
+
+.ERROR: .SILENT
+	if ! which $(GMAKE) > /dev/null; then \
+		echo "GNU Make is required!"; \
+	fi