Move platform-specific code under build tags; fix corresponding tests

ffmpeg_linux_test.go

@@ -0,0 +1,14 @@
+package ffmpeg_go
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCompileWithOptions(t *testing.T) {
+	out := Input("dummy.mp4").Output("dummy2.mp4")
+	cmd := out.Compile(SeparateProcessGroup())
+	assert.Equal(t, cmd.SysProcAttr.Pgid, 0)
+	assert.True(t, cmd.SysProcAttr.Setpgid)
+}