Fixed for MacOS

[aaronjeline]

Mac OS sets macros for string functions and has _FORTIFY_SOURCE set to 2.

[sulekhark]

Hi @aaronjeline, thank you for your work on this and for submitting this PR. It will be great if the following review comments are addressed:

1. The comments on lines 54 thro' 61 need to be updated stating why the definitions are necessary in the macOS environment in spite of FORTIFY_SOURCE being greater than 0.
2. The fix should be applicable only in the macOS environment when FORTIFY_SOURCE == 2. It should not change definitions in non-macOS environments when FORTIFY_SOURCE ==2.
3. The same fix is also applicable to some definitions in stdio_checked.h.

[dtarditi]

@aaronjeline, thanks for the changes! Could you address the pull request feedback so that we can commit this?