Apply min macOS version as 10.11 like the app store packaging

This suppports most computers sold in the last 12 years
and goes beyond Apple support plans.

Fixes fyne-io#886

cmd/fyne/commands/build.go

@@ -20,18 +20,23 @@ func (b *builder) build() error {
 		goos = targetOS()
 	}
 
-	args := []string{}
+	args := []string{"build"}
+	env := os.Environ()
+	env = append(env, "CGO_ENABLED=1") // in case someone is trying to cross-compile...
+
 	if goos == "windows" {
 		if b.release {
-			args = append(args, "build", "-ldflags", "-s -w -H=windowsgui")
+			args = append(args, "-ldflags", "-s -w -H=windowsgui")
 		} else {
-			args = append(args, "build", "-ldflags", "-H=windowsgui")
+			args = append(args, "-ldflags", "-H=windowsgui")
 		}
 	} else {
+		if goos == "darwin" {
+			env = append(env, "CGO_CFLAGS=-mmacosx-version-min=10.11")
+			env = append(env, "CGO_LDFLAGS=-mmacosx-version-min=10.11")
+		}
 		if b.release {
-			args = append(args, "build", "-ldflags", "-s -w")
-		} else {
-			args = append(args, "build")
+			args = append(args, "-ldflags", "-s -w")
 		}
 	}
 
@@ -46,9 +51,6 @@ func (b *builder) build() error {
 
 	cmd := exec.Command("go", args...)
 	cmd.Dir = b.srcdir
-	env := os.Environ()
-	env = append(env, "CGO_ENABLED=1") // in case someone is trying to cross-compile...
-
 	if goos != "ios" && goos != "android" {
 		env = append(env, "GOOS="+goos)
 	}