Merge #149: Avoid -Wundef compiler warnings

d450fbf Avoid `-Wundef` compiler warnings (Hennadii Stepanov)

Pull request description:

  Fixes bitcoin/bitcoin#30975 (comment):
  > On my own macOS 15.3 with Xcode 16.2 machine building fails:
  >
  > ```
  > git clean -dfx
  > cmake -B build -DENABLE_IPC=ON
  > cmake --build build
  > ...
  > /Users/sjors/dev/bitcoin/src/ipc/libmultiprocess/src/mp/util.cpp:22:5: error: '__linux__' is not defined, evaluates to 0 [-Werror,-Wundef]
  >    22 | #if __linux__
  >       |     ^
  > /Users/sjors/dev/bitcoin/src/ipc/libmultiprocess/src/mp/util.cpp:62:5: error: '__linux__' is not defined, evaluates to 0 [-Werror,-Wundef]
  >    62 | #if __linux__
  >       |     ^
  > 2 errors generated.
  > ```

ACKs for top commit:
  ryanofsky:
    Code review ACK d450fbf

Tree-SHA512: db7de0f43d568854d8ad1c88370de28d3d96bd55ca745852ef0adb51f427fb7f6f675af1a2baa8278c3ebcddcc27edf3f0c7c364b98ffc1d01c30f10a48bcbb8

Author: ryanofsky

src/mp/util.cpp

@@ -19,7 +19,7 @@
 #include <thread>
 #include <unistd.h>
 
-#if __linux__
+#ifdef __linux__
 #include <syscall.h>
 #endif
 
@@ -59,7 +59,7 @@ std::string ThreadName(const char* exe_name)
 
     // Prefer platform specific thread ids over the standard C++11 ones because
     // the former are shorter and are the same as what gdb prints "LWP ...".
-#if __linux__
+#ifdef __linux__
     buffer << syscall(SYS_gettid);
 #elif defined(HAVE_PTHREAD_THREADID_NP)
     uint64_t tid = 0;