Support building on FreeBSD

[dveeden]

Hi,

I tried building confluentinc/cli on FreeBSD 13.1-RELEASE-p3 and found this:

The Makefile is GNU Make specific (not very unusual). The README.md says to use make deps and make build and the Makefile also calls make. Replacing make with gmake in both the README.md and Makefile fixes that. The gmake command is also available on Linux so this should make this more portable without requiring FreeBSD specific things.

For the cgo dependency for rdkafa I needed to do a pkg install librdkafka to get librdkafka 1.9.2.

Then to build I used gmake CGO_CFLAGS=`pkg-config --cflags rdkafka` CGO_LDFLAGS=`pkg-config --libs rdkafka` TAGS='dynamic' build. Afterwards I noticed that building with gmake TAGS='dynamic' build also works, but I'm not sure if that also works for fresh builds.

% go version -m ./dist/confluent_freebsd_amd64_v1/confluent | grep -vw dep
./dist/confluent_freebsd_amd64_v1/confluent: go1.19.5
	path	command-line-arguments
	build	-asmflags=all=-trimpath=/home/dveeden/git
	build	-compiler=gc
	build	-gcflags=all=-trimpath=/home/dveeden/git
	build	-ldflags="-s -w -X main.version=v3.0.0-dirty-dveeden -X main.commit=83072d47 -X main.date=2023-01-26T08:38:18Z"
	build	-tags=dynamic
	build	CGO_ENABLED=1
	build	CGO_CFLAGS=
	build	CGO_CPPFLAGS=
	build	CGO_CXXFLAGS=
	build	CGO_LDFLAGS=
	build	GOARCH=amd64
	build	GOOS=freebsd
	build	GOAMD64=v1
% file ./dist/confluent_freebsd_amd64_v1/confluent
./dist/confluent_freebsd_amd64_v1/confluent: ELF 64-bit LSB executable, x86-64, version 1 (FreeBSD), dynamically linked, interpreter /libexec/ld-elf.so.1, for FreeBSD 13.1, FreeBSD-style, Go BuildID=nbtGuL4rOvzdWcchfu4k/FWCY7oI7U3W9T22Jk4pU/SjSsSKJdis-nENQXiNjp/mXRzoVc2_k0MVOozlSpq, stripped
% ./dist/confluent_freebsd_amd64_v1/confluent version
confluent - Confluent CLI

Version:     v3.0.0-dirty-dveeden
Git Ref:     83072d47
Build Date:  2023-01-26T08:38:18Z
Go Version:  go1.19.5 (freebsd/amd64)
Development: true