Fix syntax for sh (get rid of bash-isms)

--- libbeat/scripts/Makefile.orig	2019-10-15 21:08:29 UTC
+++ libbeat/scripts/Makefile
@@ -35,7 +35,7 @@ GOFILES = $(shell find . -type f -name '*.go')
 GOFILES_NOVENDOR = $(shell find . -type f -name '*.go' -not -path "*/vendor/*")
 GOFILES_ALL = $(GOFILES) $(shell find $(ES_BEATS) -type f -name '*.go')
 GOPACKAGES_STRESSTESTS=$(shell find . -name '*.go' | xargs awk 'FNR>1 {nextfile} /\+build.*stresstest/ {print FILENAME; nextfile}' | xargs dirname | uniq)
-SHELL=bash
+SHELL=sh
 ES_HOST?="elasticsearch"
 PWD=$(shell pwd)
 BUILD_DIR?=$(shell pwd)/build
@@ -108,7 +108,7 @@ ${BEAT_NAME}.test: $(GOFILES_ALL)
 .PHONY: crosscompile
 crosscompile: ## @build Cross-compile beat for the OS'es specified in GOX_OS variable. The binaries are placed in the build/bin directory.
 crosscompile: $(GOFILES)
-ifneq ($(shell [[ $(BEAT_NAME) == journalbeat ]] && echo true ),true)
+ifneq ($(shell [ $(BEAT_NAME) == journalbeat ] && echo true ),true)
 	go get github.com/mitchellh/gox
 	mkdir -p ${BUILD_DIR}/bin
 	gox -output="${BUILD_DIR}/bin/{{.Dir}}-{{.OS}}-{{.Arch}}" -os="$(strip $(GOX_OS))" -osarch="$(strip $(GOX_OSARCH))" ${GOX_FLAGS}
@@ -338,7 +338,7 @@ ifeq ($(BEAT_REF_YAML),true)
 	@chmod 0640 ${BEAT_NAME}.reference.yml
 endif
 
-ifneq ($(shell [[ $(BEAT_NAME) == libbeat || $(BEAT_NAME) == metricbeat ]] && echo true ),true)
+ifneq ($(shell [ $(BEAT_NAME) == libbeat -o $(BEAT_NAME) == metricbeat ] && echo true ),true)
 	mkdir -p include
 	go run  ${ES_BEATS}/dev-tools/cmd/asset/asset.go -license $(LICENSE) -pkg include -in fields.yml -out include/fields.go $(BEAT_NAME)
 endif