feat(python-build): Add PACKAGE_LDFLAGS to support custom LDFLAGS per package

[cfxegbert]

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• My PR addresses the following pyenv issue (if any)
  • Closes Add PACKAGE_LDFLAGS to python-build #2926

Description

• Here are some details about my PR
  Adds PACKAGE_LDFLAGS to be able to pass LDFLAGS separately to readline and python

Tests

• My PR adds the following unit tests (if any)

[native-api]

Wow. I was sure this was already available.

Could you adjust the tests, too? We can probably test this in the same test that tests PACKAGE_CFLAGS.

[native-api]

We can probably test this in the same test that tests PACKAGE_CFLAGS.

...And my memory fails me here, too: there is no such test.
There is however "yaml is installed for python" in build.bats which simulates a build of a dependency package and shows its build flags -- this can be taken as a base.

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It's also worth documenting the new envvar.