m1cpu: add CGO build tags and document in README

README.md

diff --git a/README.md b/README.md
index b01bf86..4cdfb70 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,11 @@
 }
 ```
 
+# CGO
+
+This package requires the use of CGO. Extracting the CPU properties is done via
+Apple's IOKit framework, which is accessible only through system C libraries.
+
 # License
 
 Open source under the [MPL](LICENSE)

cpu.go

diff --git a/cpu.go b/cpu.go
index ed29b3e..ebd0b9d 100644
--- a/cpu.go
+++ b/cpu.go
@@ -1,4 +1,4 @@
-//go:build darwin && arm64
+//go:build darwin && arm64 && cgo
 
 package m1cpu
 

cpu_test.go

diff --git a/cpu_test.go b/cpu_test.go
index 769cade..1edc836 100644
--- a/cpu_test.go
+++ b/cpu_test.go
@@ -1,4 +1,4 @@
-//go:build darwin && arm64
+//go:build darwin && arm64 && cgo
 
 package m1cpu
 

incompatible.go

diff --git a/incompatible.go b/incompatible.go
index 28412b8..76c1d8b 100644
--- a/incompatible.go
+++ b/incompatible.go
@@ -1,4 +1,4 @@
-//go:build !darwin || !arm64
+//go:build !darwin || !arm64 || !cgo
 
 package m1cpu