Fix several usability problems in the Makefile

Allow "make run" and "make gdb" to be used on Linux, not just
non-Linux.

Allow the user to set the go binary from the commandline.

Pass GOROOT in to make different Go installs use the right
Go root.

Check that xorriso is installed, because if it is not, grub-mkrescue
will silently exit.

Makefile

@@ -12,8 +12,12 @@ export SHELL := /bin/bash -o pipefail
 LD := ld
 AS := nasm
 
+# If your go is called something else set it on the commandline, like
+# this: make run GO=go1.8
+GO ?= go
 GOOS := linux
 GOARCH := amd64
+GOROOT := $(shell $(GO) env GOROOT)
 
 LD_FLAGS := -n -T $(BUILD_DIR)/linker.ld -static --no-ld-generated-unwind-info
 AS_FLAGS := -g -f elf64 -F dwarf -I arch/$(ARCH)/asm/
@@ -36,12 +40,13 @@ go.o:
 	@mkdir -p $(BUILD_DIR)
 
 	@echo "[go] compiling go sources into a standalone .o file"
-	@GOARCH=$(GOARCH) GOOS=$(GOOS) go build -n 2>&1 | sed \
+	@GOARCH=$(GOARCH) GOOS=$(GOOS) $(GO) build -n 2>&1 | sed \
 	    -e "1s|^|set -e\n|" \
 	    -e "1s|^|export GOOS=$(GOOS)\n|" \
 	    -e "1s|^|export GOARCH=$(GOARCH)\n|" \
+	    -e "1s|^|export GOROOT=$(GOROOT)\n|" \
 	    -e "1s|^|WORK='$(BUILD_ABS_DIR)'\n|" \
-	    -e "1s|^|alias pack='go tool pack'\n|" \
+	    -e "1s|^|alias pack='$(GO) tool pack'\n|" \
 	    -e "/^mv/d" \
 	    -e "s|-extld|-tmpdir='$(BUILD_ABS_DIR)' -linkmode=external -extldflags='-nostartfiles -nodefaultlibs -nostdlib -r' -extld|g" \
 	    | sh 2>&1 | sed -e "s/^/  | /g"
@@ -59,6 +64,9 @@ binutils_version_check:
 	@echo "[binutils] checking that installed objcopy version is >= $(MIN_OBJCOPY_VERSION)"
 	@if [ "$(HAVE_VALID_OBJCOPY)" != "y" ]; then echo "[binutils] error: a more up to date binutils installation is required" ; exit 1 ; fi
 
+xorriso_check:
+	@if xorriso --version >/dev/null 2>&1; then exit 0; else echo "Install xorriso via 'sudo apt install xorriso'." ; exit 1 ; fi
+
 linker_script:
 	@echo "[sed] extracting LMA and VMA from constants.inc"
 	@echo "[gcc] pre-processing arch/$(ARCH)/script/linker.ld.in"
@@ -73,7 +81,7 @@ $(BUILD_DIR)/arch/$(ARCH)/asm/%.o: arch/$(ARCH)/asm/%.s
 
 iso: $(iso_target)
 
-$(iso_target): $(kernel_target)
+$(iso_target): xorriso_check $(kernel_target)
 	@echo "[grub] building ISO kernel-$(ARCH).iso"
 
 	@mkdir -p $(BUILD_DIR)/isofiles/boot/grub
@@ -93,6 +101,8 @@ kernel:
 iso:
 	vagrant ssh -c 'cd $(VAGRANT_SRC_FOLDER); make iso'
 
+endif
+
 run: iso
 	qemu-system-$(ARCH) -cdrom $(iso_target)
 
@@ -108,7 +118,6 @@ gdb: iso
 	    -ex 'target remote localhost:1234' \
 	    -ex 'set arch i386:x86-64:intel'
 	@killall qemu-system-$(ARCH) || true
-endif
 
 clean:
 	@test -d $(BUILD_DIR) && rm -rf $(BUILD_DIR) || true
@@ -134,8 +143,8 @@ lint: lint-check-deps
 		./...
 
 lint-check-deps:
-	@go get -u gopkg.in/alecthomas/gometalinter.v1
+	@$(GO) get -u gopkg.in/alecthomas/gometalinter.v1
 	@gometalinter.v1 --install >/dev/null
 
 test:
-	go test -cover ./...
+	$(GO) test -cover ./...