maint: Detect BTF support and disable for local dev (#61)

## Which problem is this PR solving?
Updates our generate makefile target to detect if the builder supports
BTF and if not disables. This means we can remove the mac-* targets and
Dockerfile.mac file.

Also includes a small update to the generated go files to remove the old
`+build` directives after running `docker-generate`.

## Short description of the changes
- Update Makefile to detect if BTF is support, if not sets
`BPF_NO_PRESERVE_ACCESS_INDEX` env var used to generate go files from
probe
- Remove mac specific Makefile targets
- Remove Dockerfile.mac file

## How to verify that this has the expected result
Can build and run the agent locally by using `docker-build`.

DEVELOPING.md

@@ -20,8 +20,6 @@ For example, run it after changing the `tcp_event` struct in `tcp_probe.c`.
 
 When building with `make docker-build`, the generated files are included in the build but not updated locally.
 
-To run a locally-built image in Kubernetes on Docker Desktop, run `make mac-docker-build`.
-
 ## To pull a published image from ghcr
 
 Docker images are found in [`ghcr.io/honeycombio/ebpf-agent:latest`](https://github.com/honeycombio/honeycomb-ebpf-agent/pkgs/container/ebpf-agent).

Makefile

@@ -5,6 +5,13 @@ REPODIR := $(shell dirname $(realpath $(firstword $(MAKEFILE_LIST))))
 # Build the list of header directories to compile the bpf program
 BPF_HEADERS += -I${REPODIR}/bpf/headers
 
+# Disable BTF if the kernel doesn't support it (eg local dev on Docker Desktop)
+# needed until BTF is enabled for Docker Desktop
+# see https://github.com/docker/for-mac/issues/6800
+ifeq (,$(wildcard /sys/kernel/btf/vmlinux))
+	BPF_HEADERS += -DBPF_NO_PRESERVE_ACCESS_INDEX
+endif
+
 IMG_NAME ?= hny/ebpf-agent
 IMG_TAG ?= local
 
@@ -31,24 +38,6 @@ update-headers:
 	cd bpf/headers && ./update.sh
 	@echo "*** Also update bpf_tracing.h file! ***"
 
-### Local Mac Build for Kubernetes on Docker Desktop
-
-# needed until BTF is enabled for Docker Desktop
-# see https://github.com/docker/for-mac/issues/6800
-
-.PHONY: mac-generate
-mac-generate: export CFLAGS := $(BPF_HEADERS) -DBPF_NO_PRESERVE_ACCESS_INDEX
-mac-generate:
-	go generate ./...
-
-.PHONY: mac-build
-mac-build: mac-generate
-	CGO_ENABLED=0 GOOS=linux go build -o hny-ebpf-agent main.go
-
-.PHONY: mac-docker-build
-mac-docker-build:
-	docker build --tag $(IMG_NAME):$(IMG_TAG) -f Dockerfile.mac .
-
 ### Testing targets
 
 # deploy ebpf agent daemonset to already-running cluster with env vars from .env file