Propagate the Position Independent Code flag

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

In CMake, if the caller has set CMAKE_POSITION_INDEPENDENT_CODE, that needs to be propagated to the special early build of the dependencies like boringssl, so that the libraries can link together properly.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Building the Unity SDK on Linux with boringssl enabled.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[jonsimantov]

LGTM, I'd like to see an expanded matrix test run, but if you need this in for Unity then go ahead and merge.

[github-actions]

Integration test with FLAKINESS (succeeded after retry)

Requested by @a-maurice on commit 2037576
Last updated: Fri May 13 13:09 PDT 2022
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT

Add flaky tests to go/fpl-cpp-flake-tracker