Add custom build backend to support build args (#328)

This implements a custom build backend, inspired by the
[solution used by Pillow.](https://github.com/python-pillow/Pillow/pull/7171)

install-option is deprecated and was removed with pip 23.1. The
comonly accepted solution seems to be to define a custom build
backend for now
https://github.com/pypa/setuptools/issues/2491#issuecomment-1589764230

This commit changes the usage from `--install-option` to `--config-settings`.

_custom_build/backend.py

@@ -0,0 +1,31 @@
+import sys
+
+from setuptools.build_meta import *  # noqa: F401, F403
+from setuptools.build_meta import build_wheel
+
+backend_class = build_wheel.__self__.__class__
+
+
+class _CustomBuildMetaBackend(backend_class):
+    def run_setup(self, setup_script="setup.py"):
+        if self.config_settings:
+            flags = []
+            if self.config_settings.get("force-bundled-libcapnp"):
+                flags.append("--force-bundled-libcapnp")
+            if self.config_settings.get("force-system-libcapnp"):
+                flags.append("--force-system-libcapnp")
+            if self.config_settings.get("libcapnp-url"):
+                flags.append("--libcapnp-url")
+                flags.append(self.config_settings["libcapnp-url"])
+            if flags:
+                sys.argv = sys.argv[:1] + ["build_ext"] + flags + sys.argv[1:]
+        return super().run_setup(setup_script)
+
+    def build_wheel(
+        self, wheel_directory, config_settings=None, metadata_directory=None
+    ):
+        self.config_settings = config_settings
+        return super().build_wheel(wheel_directory, config_settings, metadata_directory)
+
+
+build_wheel = _CustomBuildMetaBackend().build_wheel