Makefile: use bash

This fixes `make xbuild` on my Debian system

Makefile

@@ -1,9 +1,7 @@
-version := v0.1.0
+version := v0.1.1
 platforms := linux_386 linux_amd64 darwin_amd64 windows_386 windows_amd64
+SHELL := /bin/bash
 
-# Requires gnu make 3.82+
-# (On macos use brew install make and execute as 'gmake')
-.ONESHELL:
 .PHONY: build
 build:
 	go build -o gsheet ./cmd/gsheet/
@@ -14,8 +12,8 @@ test:
 
 .PHONY: xbuild
 xbuild:
-	for platform in ${platforms}; do
-		pair=($${platform/_/ })
-		GOARCH=$${pair[1]} GOOS=$${pair[0]} go build -ldflags '-s -w -X main.version=${version}' -o build/$$platform/gsheet ./cmd/gsheet/
-		zip -j -r build/$${platform}.zip build/$${platform}/
+	for platform in ${platforms}; do \
+		pair=($${platform/_/ }) ;\
+		GOARCH=$${pair[1]} GOOS=$${pair[0]} go build -ldflags '-s -w -X main.version=${version}' -o build/$$platform/gsheet ./cmd/gsheet/ ;\
+		zip -j -r build/$${platform}.zip build/$${platform}/ ;\
 	done