context: add linkmode and buildmode

Fixes #340

No accessors to change linkmode and buildmode because we don't
support anything except buildmode=exe, and linkmode defaults to
automatic.

context.go

@@ -37,6 +37,8 @@ type Context struct {
 
 	gcflags []string // flags passed to the compiler
 	ldflags []string // flags passed to the linker
+
+	linkmode, buildmode string // link and build modes
 }
 
 // GOOS configures the Context to use goos as the target os.
@@ -88,9 +90,10 @@ func (p *Project) NewContext(opts ...func(*Context) error) (*Context, error) {
 		GcToolchain(),
 	}
 	ctx := Context{
-		Project: p,
-		workdir: mktmpdir(),
-		pkgs:    make(map[string]*Package),
+		Project:   p,
+		workdir:   mktmpdir(),
+		pkgs:      make(map[string]*Package),
+		buildmode: "exe",
 	}
 
 	for _, opt := range append(defaults, opts...) {

gc.go

@@ -85,7 +85,7 @@ func (t *gcToolchain) Ld(pkg *Package, searchpaths []string, outfile, afile stri
 	}
 	args = append(args, "-extld="+gcc())
 	if gc15 {
-		args = append(args, "-buildmode=exe")
+		args = append(args, "-buildmode", pkg.buildmode)
 	}
 	args = append(args, afile)
 	if err := mkdir(filepath.Dir(outfile)); err != nil {