Add env var to control stripping debug info (#507)

Before this commit, debug info is stripped unconditionally on Linux.

This commit adds an environment variable `UJSON_BUILD_NO_STRIP` that disables this behavior. This is helpful for distribution packagers who would otherwise have to patch `setup.py` to prevent stripping.

setup.py

@@ -1,12 +1,19 @@
 import platform
 from glob import glob
+from os import environ
 
 from setuptools import Extension, setup
 
 dconv_source_files = glob("./deps/double-conversion/double-conversion/*.cc")
 dconv_source_files.append("./lib/dconv_wrapper.cc")
 
-strip_flags = ["-Wl,--strip-all"] if platform.system() == "Linux" else []
+if platform.system() == "Linux" and environ.get("UJSON_BUILD_NO_STRIP", "0") not in (
+    "1",
+    "True",
+):
+    strip_flags = ["-Wl,--strip-all"]
+else:
+    strip_flags = []
 
 module1 = Extension(
     "ujson",