main: use GOPATH from the environment

Be more compatible with the Go toolchain by setting GOPATH in the same
way. This makes it possible to flash and run examples from the standard
GOPATH instead of only from the source tree.

main.go

@@ -22,6 +22,7 @@ func Compile(pkgName, outpath string, spec *TargetSpec, printIR, dumpSSA, debug
 		Debug:     debug,
 		DumpSSA:   dumpSSA,
 		RootDir:   sourceDir(),
+		GOPATH:    getGopath(),
 		BuildTags: spec.BuildTags,
 	}
 	c, err := compiler.NewCompiler(pkgName, config)
@@ -198,6 +199,7 @@ func Flash(pkgName, target, port string, printIR, dumpSSA, debug bool, printSize
 func Run(pkgName string) error {
 	config := compiler.Config{
 		RootDir: sourceDir(),
+		GOPATH:  getGopath(),
 	}
 	c, err := compiler.NewCompiler(pkgName, config)
 	if err != nil {