bpo-36231: Support building on macOS without /usr/include (GH-13773)

Misc/NEWS.d/next/macOS/2019-06-03-05-49-49.bpo-36231.RfmW_p.rst

+Support building Python on macOS without /usr/include installed. As of macOS
+10.14, system header files are only available within an SDK provided by
+either the Command Line Tools or the Xcode app.

setup.py

@@ -125,19 +125,57 @@ def sysroot_paths(make_vars, subdirs):
                 break
     return dirs
 
+MACOS_SDK_ROOT = None
 
 def macosx_sdk_root():
+    """Return the directory of the current macOS SDK.
+
+    If no SDK was explicitly configured, call the compiler to find which
+    include files paths are being searched by default.  Use '/' if the
+    compiler is searching /usr/include (meaning system header files are
+    installed) or use the root of an SDK if that is being searched.
+    (The SDK may be supplied via Xcode or via the Command Line Tools).
+    The SDK paths used by Apple-supplied tool chains depend on the
+    setting of various variables; see the xcrun man page for more info.
     """
-    Return the directory of the current OSX SDK,
-    or '/' if no SDK was specified.
-    """
+    global MACOS_SDK_ROOT
+
+    # If already called, return cached result.
+    if MACOS_SDK_ROOT:
+        return MACOS_SDK_ROOT
+
     cflags = sysconfig.get_config_var('CFLAGS')
     m = re.search(r'-isysroot\s+(\S+)', cflags)
-    if m is None:
-        sysroot = '/'
+    if m is not None:
+        MACOS_SDK_ROOT = m.group(1)
     else:
-        sysroot = m.group(1)
-    return sysroot
+        MACOS_SDK_ROOT = '/'
+        cc = sysconfig.get_config_var('CC')
+        tmpfile = '/tmp/setup_sdk_root.%d' % os.getpid()
+        try:
+            os.unlink(tmpfile)
+        except:
+            pass
+        ret = os.system('%s -E -v - </dev/null 2>%s 1>/dev/null' % (cc, tmpfile))
+        in_incdirs = False
+        try:
+            if ret >> 8 == 0:
+                with open(tmpfile) as fp:
+                    for line in fp.readlines():
+                        if line.startswith("#include <...>"):
+                            in_incdirs = True
+                        elif line.startswith("End of search list"):
+                            in_incdirs = False
+                        elif in_incdirs:
+                            line = line.strip()
+                            if line == '/usr/include':
+                                MACOS_SDK_ROOT = '/'
+                            elif line.endswith(".sdk/usr/include"):
+                                MACOS_SDK_ROOT = line[:-12]
+        finally:
+            os.unlink(tmpfile)
+
+    return MACOS_SDK_ROOT
 
 
 def is_macosx_sdk_path(path):