Include CPPFLAGS when searching for libraries

This adds CPPFLAGS to the list of environment variables (currently
CFLAGS and LDFLAGS) searched when looking for includes and library
directories, as CPPFLAGS may also include -I options.

setup.py

@@ -386,8 +386,8 @@ def build_extensions(self):
             _add_directory(library_dirs, lib_root)
             _add_directory(include_dirs, include_root)
 
-        # respect CFLAGS/LDFLAGS
-        for k in ("CFLAGS", "LDFLAGS"):
+        # respect CFLAGS/CPPFLAGS/LDFLAGS
+        for k in ("CFLAGS", "CPPFLAGS", "LDFLAGS"):
             if k in os.environ:
                 for match in re.finditer(r"-I([^\s]+)", os.environ[k]):
                     _add_directory(include_dirs, match.group(1))