Turn boringssl on for Linux, with C++ fix

[a-maurice]

Description

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The problem with boringssl on linux was because the C++ repo was not passing along the PIC flag correctly. This will be fixed with firebase/firebase-cpp-sdk#957, at which point we can use boringssl on linux.

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Testing

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Type of Change

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• 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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