add note about MacOS malloc warnings for tsan build

docs/debugging.md

@@ -316,6 +316,21 @@ PYTHON_GIL=0 python -c "import sys; print(sys._is_gil_enabled())"
 cd ..
 ```
 
+!!! note
+    On MacOS, you may see messages like this when you start Python:
+
+    ```
+    python(7027,0x1f6dfc240) malloc: nano zone abandoned due to inability to reserve vm space.
+    ```
+
+    This message is being emmitted by the MacOS malloc implementation. As
+    [explained
+    here](https://stackoverflow.com/questions/64126942/malloc-nano-zone-abandoned-due-to-inability-to-preallocate-reserved-vm-space),
+    this happens for any program compiled with thread sanitizer on MacOS and can
+    be safely ignored by setting the `MallocNanoZone` environment variable to
+    0\. You should only set this in session you are running thread sanitizer
+    under, as this setting will slow down other programs that allocate memory.
+
 ### Compile NumPy with TSAN
 
 - Get the source code (for example, the `main` branch)
@@ -336,13 +351,7 @@ python -m pip install -U git+https://github.com/cython/cython
 - Build the package
 
 ```bash
-export CC=clang-18
-export CXX=clang++-18
-export CFLAGS=-fsanitize=thread
-export CXXFLAGS=-fsanitize=thread
-export LDFLAGS=-fsanitize=thread
-
-python -m pip install -v . --no-build-isolation
+CC=clang-18 CXX=clang++-18 python -m pip install -v . --no-build-isolation -C'setup-args=-Db_sanitize=thread
 ```
 
 ### Useful TSAN options