Add before_build option, that runs a shell command before 'pip wheel'

test/03_before_build/environment.json

@@ -0,0 +1,3 @@
+{
+    "CIBW_BEFORE_BUILD": "{python} -c \"import sys; open('version.txt', 'w').write(sys.version)\""
+}

test/03_before_build/setup.py

@@ -0,0 +1,17 @@
+from setuptools import setup, Extension
+import sys
+
+# here we assert that the Python version as written to version.txt in the CIBW_BEFORE_BUILD step
+# is the same one as is currently running.
+sys.stderr.write('sys.argv \n' + str(sys.argv))
+if sys.argv[-1] != '--name':
+    with open('version.txt') as f:
+        stored_version = f.read()
+    print('version is', stored_version)
+    assert stored_version == sys.version
+
+setup(
+    name="spam",
+    ext_modules=[Extension('spam', sources=['spam.c'])],
+    version="0.1.0",
+)

test/03_before_build/spam.c

@@ -0,0 +1,48 @@
+#include <Python.h>
+
+static PyObject *
+spam_system(PyObject *self, PyObject *args)
+{
+    const char *command;
+    int sts;
+
+    if (!PyArg_ParseTuple(args, "s", &command))
+        return NULL;
+    sts = system(command);
+    return PyLong_FromLong(sts);
+}
+
+/* Module initialization */
+
+#if PY_MAJOR_VERSION >= 3
+    #define MOD_INIT(name) PyMODINIT_FUNC PyInit_##name(void)
+    #define MOD_DEF(m, name, doc, methods, module_state_size) \
+        static struct PyModuleDef moduledef = { \
+            PyModuleDef_HEAD_INIT, name, doc, module_state_size, methods, }; \
+        m = PyModule_Create(&moduledef);
+    #define MOD_RETURN(m) return m;
+#else
+    #define MOD_INIT(name) PyMODINIT_FUNC init##name(void)
+    #define MOD_DEF(m, name, doc, methods, module_state_size) \
+        m = Py_InitModule3(name, methods, doc);
+    #define MOD_RETURN(m) return;
+#endif
+
+static PyMethodDef module_methods[] = {
+    {"system", (PyCFunction)spam_system, METH_VARARGS, 
+     "Execute a shell command."},
+    {NULL}  /* Sentinel */
+};
+
+MOD_INIT(spam)
+{
+    PyObject* m;
+
+    MOD_DEF(m, 
+            "spam", 
+            "Example module", 
+            module_methods,
+            -1)
+
+    MOD_RETURN(m)
+}

test/03_before_build/version.txt

@@ -0,0 +1,2 @@
+3.6.0 (default, Feb  7 2017, 23:55:32) 
+[GCC 4.8.2 20140120 (Red Hat 4.8.2-15)]