Support dynamically linking against system double-conversion library

[musicinmybrain]

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.

This is particularly useful to Linux distribution packagers who would otherwise need a patch to unbundle double-conversion. (It may still be necessary to rm -rf deps/double-conversion; I have not checked, as Fedora Linux would require this anyway.)

Fixes #375

Changes proposed in this pull request:

• Allow overriding default include path/paths for double-conversion with environment variable UJSON_BUILD_DC_INCLUDES. This allows multiple whitespace-delimited paths but as a consequence doesn’t support paths containing whitespace, which is probably acceptable for the intended uses. For example, Linux distributions building against a system copy of double-conversion will want something like UJSON_BUILD_DC_INCLUDES=/usr/include/double-conversion.
• Allow adding linker flags for double-conversion with environment variable UJSON_BUILD_DC_LIBS; by default there are still none. For example, Linux distributions building against a system copy of double-conversion will want something like UJSON_BUILD_DC_LIBS=-ldouble-conversion.

[bwoodsend]

Could you add a little Build Options section to the end of the readme to illustrate to others how to link against a system double-conversion?

[musicinmybrain]

Yes. I can add that to this PR.

[musicinmybrain]

Made some changes based on feedback. Ensured the bundled double-conversion is not built if UJSON_BUILD_DC_LIBS is set to a non-empty value. All further feedback is welcome.

[codecov-commenter]

Codecov Report

Merging #508 (4688286) into main (7f269a4) will not change coverage.
The diff coverage is n/a.

❗ Current head 4688286 differs from pull request most recent head 9ee6cbb. Consider uploading reports for the commit 9ee6cbb to get more accurate results

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[bwoodsend]

Do you need ujson to release this before you can go back to your work on the Fedora RPM?

[musicinmybrain]

Do you need ujson to release this before you can go back to your work on the Fedora RPM?

Not really—we can easily carry it as a downstream patch for now.