Fix wrong libpython being linked to in MacOS

[native-api]

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• My PR addresses the following pyenv issue (if any)
  • Closes Installing 2.7.18 on a macOS with existing system 2.7.16 will actually yield 2.7.16 #2578

Description

• Here are some details about my PR

The comment at

pyenv/plugins/python-build/bin/python-build

Line 2210 in bd618ea

# The ld on Darwin embeds the full paths to each dylib by default

said that rpath does not need to be set in MacOS.
Looks like this is not the case anymore: bin/python was linking to the stock Apple's libpython without it.

@yyuu , as the person who authored 0d85193, do you remember anything? Maybe rpath only needs to be added to PYTHON_LDFLAGS?

Tests

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