Support dynamically linking against system double-conversion library (#508)

Add env vars to build with system double-conversion. Fixes #376 and is useful to Linux distribution packagers.

New environment variables UJSON_BUILD_DC_INCLUDES and UJSON_BUILD_DC_LIBS allow overriding the include path for double-conversion and adding linker flags for an external double-conversion library. They should generally be used together.

README.md

@@ -124,3 +124,39 @@ Linux 5.0.0-1032-azure x86_64 #34-Ubuntu SMP Mon Feb 10 19:37:25 UTC 2020
 | Complex object                                                                |            |            |            |            |            |
 | encode                                                                        |        533 |        582 |            |        408 |        431 |
 | decode                                                                        |        466 |        454 |            |        154 |        164 |
+
+## Build options
+
+For those with particular needs, such as Linux distribution packagers, several
+build options are provided in the form of environment variables.
+
+### Debugging symbols
+
+#### UJSON_BUILD_NO_STRIP
+
+By default, debugging symbols are stripped on Linux platforms. Setting this
+environment variable with a value of `1` or `True` disables this behavior.
+
+### Using an external or system copy of the double-conversion library
+
+These two environment variables are typically used together, something like:
+
+```sh
+export UJSON_BUILD_DC_INCLUDES='/usr/include/double-conversion'
+export UJSON_BUILD_DC_LIBS='-ldouble-conversion'
+```
+
+Users planning to link against an external shared library should be aware of
+the ABI-compatibility requirements this introduces when upgrading system
+libraries or copying compiled wheels to other machines.
+
+#### UJSON_BUILD_DC_INCLUDES
+
+One or more directories, delimited by `os.pathsep` (same as the `PATH`
+environment variable), in which to look for `double-conversion` header files;
+the default is to use the bundled copy.
+
+#### UJSON_BUILD_DC_LIBS
+
+Compiler flags needed to link the `double-conversion` library; the default
+is to use the bundled copy.

setup.py

@@ -1,10 +1,22 @@
 import platform
+import shlex
 from glob import glob
-from os import environ
+from os import environ, pathsep
 
 from setuptools import Extension, setup
 
-dconv_source_files = glob("./deps/double-conversion/double-conversion/*.cc")
+dconv_includes = [
+    dir
+    for dir in environ.get(
+        "UJSON_BUILD_DC_INCLUDES",
+        "./deps/double-conversion/double-conversion",
+    ).split(pathsep)
+    if dir
+]
+dconv_libs = shlex.split(environ.get("UJSON_BUILD_DC_LIBS", ""))
+dconv_source_files = []
+if not dconv_libs:
+    dconv_source_files.extend(glob("./deps/double-conversion/double-conversion/*.cc"))
 dconv_source_files.append("./lib/dconv_wrapper.cc")
 
 if platform.system() == "Linux" and environ.get("UJSON_BUILD_NO_STRIP", "0") not in (
@@ -25,9 +37,9 @@ module1 = Extension(
         "./lib/ultrajsonenc.c",
         "./lib/ultrajsondec.c",
     ],
-    include_dirs=["./python", "./lib", "./deps/double-conversion/double-conversion"],
+    include_dirs=["./python", "./lib"] + dconv_includes,
     extra_compile_args=["-D_GNU_SOURCE"],
-    extra_link_args=["-lstdc++", "-lm"] + strip_flags,
+    extra_link_args=["-lstdc++", "-lm"] + dconv_libs + strip_flags,
 )
 
 with open("python/version_template.h") as f: