Makefile: Add the install-tools target

ith a fresh checked out k3d tree without required tools installed, Any make command gives the following error message:

    Makefile:71: *** recipe commences before first target.  Stop.

Basically, the section around line 71 are not properly formed makefile rules. This error is hidden when required tools
are installed. The error can be re-exposed with the "make extra-clean" command.

The install-tools target is intended to be invoked when installing the development tree for the fist time.

Makefile

@@ -34,7 +34,7 @@ export GO111MODULE=on
 # go source files, ignore vendor directory
 SRC = $(shell find . -type f -name '*.go' -not -path "./*/*")
 
-.PHONY: all build build-cross clean fmt simplify check extra-clean
+.PHONY: all build build-cross clean fmt simplify check extra-clean install-tools
 
 all: clean fmt check build
 
@@ -67,9 +67,10 @@ check:
 HAS_GOX := $(shell command -v gox 2> /dev/null)
 HAS_GOLANGCI  := $(shell command -v golangci-lint 2> /dev/null)
 
+install-tools:
 ifndef HAS_GOX
-	go get -u $(PKG_GOX)
+	(export GO111MODULE=off; go get -u $(PKG_GOX))
 endif
 ifndef HAS_GOLANGCI
-	go get -u $(PKG_GOLANGCI_LINT)
+	(export GO111MODULE=off; go get -u $(PKG_GOLANGCI_LINT))
 endif

README.md

@@ -17,7 +17,7 @@ This repository is based on [@zeerorg](https://github.com/zeerorg/)'s [zeerorg/k
 
 You have several options there:
 
-- use the install script to grab the latest release: 
+- use the install script to grab the latest release:
   - wget: `wget -q -O - https://raw.githubusercontent.com/rancher/k3d/master/install.sh | bash`
   - curl: `curl -s https://raw.githubusercontent.com/rancher/k3d/master/install.sh | bash`
 - Grab a release from the [release tab](https://github.com/rancher/k3d/releases) and install it yourself.
@@ -29,7 +29,9 @@ or...
 
 1. Clone this repo, e.g. via `go get -u github.com/rancher/k3d`
 2. Inside the repo run
-   - `make` to build for your current system
+   - 'make install-tools' to make sure required go packages are installed
+3. Inside the repo run
+   - `make build` to build for your current system
    - `go install` to install it to your `GOPATH`
    - `make build-cross` to build for all systems