Set CIBUILDWHEEL environment variable to 1 during build

README.md

@@ -31,6 +31,17 @@ Usage
 
 `cibuildwheel` is not intended to run on your development machine. It will try to install packages globally; this is no good. Travis CI and Appveyor run their builds in isolated environments, so are ideal for this kind of script.
 
+`cibuildwheel` defines the environment variable `CIBUILDWHEEL` to the value `1` allowing projects for which the C extension is optional to make it mandatory when building wheels.
+An easy way to do it in Python 3 is through the `optional` named argument of `Extension` constructor in your `setup.py`:
+```python
+myextension = Extension(
+    "myextension",
+    ["myextension.c"],
+    optional=os.environ.get('CIBUILDWHEEL', '0') != '1',
+)
+```
+
+
 ### Minimal setup
 
 - Create a `.travis.yml` file in your repo.