Update building_and_testing.md

Author: orbeckst

docs/docs/building_and_testing.md

@@ -5,20 +5,13 @@ You can build `distopia` from source.
 ## Checking out the sources
 Get the latest sources from the repository https://github.com/MDAnalysis/distopia
 ```bash
-git clone https://github.com/MDAnalysis/distopia.git
+git clone --recurse-submodules https://github.com/MDAnalysis/distopia.git
 cd distopia
 ```
 
-We also need `highway`, which vendored as a git submodule:
-```bash
-git submodule update --init libdistopia/highway
-```
-
-For testing and benchmarking get `googlebench` and `googletest`:
-```bash
-git submodule update --init libdistopia/googlebench
-git submodule update --init libdistopia/googletest
-```
+Note that we are using git submodules for the vendored [highway](https://github.com/google/highway)
+library (in `libdistopia/highway`, release 1.3.0) and for Google tests and Google benchmarks (in 
+`libdistopia/googletest` and `libdistopia/googlebench`).
 
 
 ## Building 
@@ -37,6 +30,14 @@ If you want greater control over build and install options you can do a more man
 python setup.py install -- -DCMAKE_BUILD_TYPE=Release <etc etc>
 ```
 
+Special CMake flags
+- `DISTOPIA_USE_EXTERNAL_HWY`:
+  - Off (default) — build with the vendored highway library
+  - On — link to an installed libhwy (install with `mamba install libhwy`)
+- `DISTOPIA_BUILD_TESTS`:
+  - On (default) — build the tests for libdistopia and the benchmarks
+  - Off — do not build tests and benchmarks
+
 ## Testing
 
 Testing for distopia is done on two levels. The `distopia` python layer has `pytest` tests you can run after installing the package.