Avoid -Wundef compiler warnings #149

1. 
   hebasto commented at 9:04 am on February 4, 2025: member

   Fixes https://github.com/bitcoin/bitcoin/pull/30975#issuecomment-2633222863:

   On my own macOS 15.3 with Xcode 16.2 machine building fails:

    0git clean -dfx
    1cmake -B build -DENABLE_IPC=ON
    2cmake --build build
    3...
    4/Users/sjors/dev/bitcoin/src/ipc/libmultiprocess/src/mp/util.cpp:22:5: error: '__linux__' is not defined, evaluates to 0 [-Werror,-Wundef]
    5   22 | #if __linux__
    6      |     ^
    7/Users/sjors/dev/bitcoin/src/ipc/libmultiprocess/src/mp/util.cpp:62:5: error: '__linux__' is not defined, evaluates to 0 [-Werror,-Wundef]
    8   62 | #if __linux__
    9      |     ^
   102 errors generated.
   

2. Avoid `-Wundef` compiler warnings d450fbf63e
3. 
   Sjors commented at 9:39 am on February 4, 2025: member

   That does the trick for me locally, thanks! Will try in bitcoin/bitcoin#30975
4. Sjors referenced this in commit 7f3a3c38fb on Feb 4, 2025
5. Sjors referenced this in commit b0e1d11e8a on Feb 4, 2025
6. Armss9936 approved
7. ryanofsky approved
8. 
   ryanofsky commented at 3:41 am on February 5, 2025: collaborator

   Code review ACK d450fbf63ef4308dbeb164d9999e0c09f9ec5c91
9. ryanofsky merged this on Feb 5, 2025
10. ryanofsky closed this on Feb 5, 2025

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Contributors
hebasto Sjors Armss9936 ryanofsky