Update version to 0.3.21

Author: sdroege

CHANGELOG.md

@@ -5,6 +5,17 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.3.21] - 2021-10-22
+
+### Fixed
+
+- Tests succeed again on macOS (#122).
+
+### Changed
+
+- Improve error message in case of missing pkg-config and provide instructions
+  how it can be installed (#121).
+
 ## [0.3.20] - 2021-09-25
 
 ### Fixed

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "pkg-config"
-version = "0.3.20"
+version = "0.3.21"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 license = "MIT/Apache-2.0"
 repository = "https://github.com/rust-lang/pkg-config-rs"