CMake: Set _FORTIFY_SOURCE once

[jschueller]

Description

Avoids to redefine _FORTIFY_SOURCE twice:

[  2%] Building CXX object src/tbb/CMakeFiles/tbb.dir/address_waiter.cpp.obj
<command-line>: error: "_FORTIFY_SOURCE" redefined [-Werror]

Fixes # - issue number(s) if exists

• [x] - git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details)

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• [ ] bug fix - change that fixes an issue
• [ ] new feature - change that adds functionality
• [ ] tests - change in tests
• [ ] infrastructure - change in infrastructure and CI
• [ ] documentation - documentation update

Tests

• [ ] added - required for new features and some bug fixes
• [x] not needed

Documentation

• [ ] updated in # - add PR number
• [ ] needs to be updated
• [x] not needed

Breaks backward compatibility

• [x] Yes
• [ ] No
• [ ] Unknown

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