pass: add textflags.h if required

Closes #12

pass/textflag.go

@@ -0,0 +1,41 @@
+package pass
+
+import (
+	"github.com/mmcloughlin/avo"
+)
+
+// IncludeTextFlagHeader includes textflag.h if necessary.
+func IncludeTextFlagHeader(f *avo.File) error {
+	const textflagheader = "textflag.h"
+
+	// Check if we already have it.
+	for _, path := range f.Includes {
+		if path == textflagheader {
+			return nil
+		}
+	}
+
+	// Add it if necessary.
+	if requirestextflags(f) {
+		f.Includes = append(f.Includes, textflagheader)
+	}
+
+	return nil
+}
+
+// requirestextflags returns whether the file uses flags in the textflags.h header.
+func requirestextflags(f *avo.File) bool {
+	for _, s := range f.Sections {
+		var a avo.Attribute
+		switch s := s.(type) {
+		case *avo.Function:
+			a = s.Attributes
+		case *avo.Global:
+			a = s.Attributes
+		}
+		if a.ContainsTextFlags() {
+			return true
+		}
+	}
+	return false
+}