not just CGO_CFLAGS_ALLOW='-mfoo'. Similarly named variables control
the allowed CPPFLAGS, CXXFLAGS, FFLAGS, and LDFLAGS.

Also for security reasons, only a limited set of characters are
permitted, notably alphanumeric characters and a few symbols, such as
'.', that will not be interpreted in unexpected ways. Attempts to use
forbidden characters will get a "malformed #cgo argument" error.

	// We use the Stack's input method; no macro processing at this stage.
	tok := in.Stack.Next()
	if tok != scanner.Ident {
		in.expectText("expected identifier after # directive")
	}
	// Name is alphanumeric by definition.
	return in.Stack.Text()
}

// #define processing.
func (in *Input) define() {
	name := in.macroName()
	args, tokens := in.macroDefinition(name)
	in.defineMacro(name, args, tokens)
}

	}
	return b
}

// isSeparator reports whether the rune could mark a word boundary.
// TODO: update when package unicode captures more of the properties.
func isSeparator(r rune) bool {
	// ASCII alphanumerics and underscore are not separators
	if r <= 0x7F {
		switch {
		case '0' <= r && r <= '9':
			return false
		case 'a' <= r && r <= 'z':
			return false
		case 'A' <= r && r <= 'Z':
			return false