Fix FreeType build PyPy

PyPy doesn't have a host type, so don't pass `--host` in that case, and
hope it is correct (which has been how it was since forever anyway.)

Author: QuLogic

setupext.py

@@ -617,12 +617,15 @@ def do_custom_build(self, env):
                 },
             }
             env["CFLAGS"] = env.get("CFLAGS", "") + " -fPIC"
-            subprocess.check_call(
-                ["./configure", "--with-zlib=no", "--with-bzip2=no",
-                 "--with-png=no", "--with-harfbuzz=no", "--enable-static",
-                 "--disable-shared",
-                 "--host=" + sysconfig.get_config_var('BUILD_GNU_TYPE')],
-                env=env, cwd=src_path)
+            configure = [
+                "./configure", "--with-zlib=no", "--with-bzip2=no",
+                "--with-png=no", "--with-harfbuzz=no", "--enable-static",
+                "--disable-shared"
+            ]
+            host = sysconfig.get_config_var('BUILD_GNU_TYPE')
+            if host is not None:  # May be unset on PyPy.
+                configure.append(f"--host={host}")
+            subprocess.check_call(configure, env=env, cwd=src_path)
             if 'GNUMAKE' in env:
                 make = env['GNUMAKE']
             elif 'MAKE' in env: