native xmake toolchain definition for UE-clang (#805)

This change is meant to provide a smoother experience when working on Linux. After this change, the toolchain setup process is now simply

```bash
$ scripts/ue_build_linux/get_ue_toolchain.sh 
```

and then at config time the toolchain is automatically detected if you downloaded it to the default location or have the `UE_TOOLCHAIN_DIR` environment variable set

```bash
xmake config --mode=debug
```

Compared to the old script-based approach this configures the toolchain more precisely, avoiding leakage into unrelated build processes such as when a package manager decides to build something like Ninja locally etc.

1 files changed, 32 insertions, 36 deletions

diff --git a/README.md b/README.md
index 36e02d213..2146738ec 100644
--- a/README.md
+++ b/README.md
@@ -75,60 +75,56 @@ line.
 
 ## Building on Linux
 
-The following instructions have been collated using Ubuntu 20.04.
-
-Zen makes use of C++20 features which at the time of writing has limited
-toolchain and C++ library support. A minimum compiler version of GCC-11 or
-Clang-12 is required, along with GNU's libstdc+++-11 or newer. 
-
-The first step is to acquire a suitable compiler and C++ library. On Ubuntu 20.04
-GCC-11 is not available in the standard package repositories so Ubuntu's tool-
-chain test respository needs to be added to Apt (note that this is _not required_
-for Ubuntu 21.04 onwards);
-
-```
-sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
-```
-
-Now GCC-11 can be installed via Apt. This will also install a suitable version of
-the C++ library.
-
-```
-sudo apt install -y --no-install-recommends g++-11
-g++-11 --version
-```
-
-The easiest way to tell `xmake` to use the correct compiler version is to set
-the `CXX` environment variable.
+The following instructions have been collated using Ubuntu 24.04. Other distributions will
+likely need a slightly different set of packages installed.
 
-```
-export CXX=g++-11
+```bash
+sudo apt install -y build-essential pkg-config unzip 
 ```
 
 Install [xmake](https://xmake.io/#/getting_started).
 
-```
+It's important to use the correct version when installing, pay attention to the last argument in the command line below:
+
+```bash
 curl -fsSL https://xmake.io/shget.text | bash -s v2.9.9
 ```
 
-Clone the Zen project and tell `xmake` to use the correct GCC version.
+Clone the Zen project.
 
-```
+```bash
 git clone https://github.com/EpicGames/zen.git ~/zen/main
 cd ~/zen/main
 ```
 
-Now we are ready to build Zen.  The `-y` skips `xmake` from prompting about
-building packages.
+We recommend building using the UE Linux toolchain, to match the Unreal Editor
+setup as closely as possible. It is also the most thoroughly tested toolchain
+since this is what is used in the CI environment which produces the binaries
+which end up in the UE tree.
+
+Using other toolchains may or may not work depending on which version and 
+compiler you happen to be using.
 
+### Building with the UE Linux toolchain
+
+To build with the Unreal Engine cross-compile toolchain (producing binaries
+compatible with the VFX Reference Platform), first download the toolchain:
+
+```bash
+scripts/ue_build_linux/get_ue_toolchain.sh
 ```
-xmake config -y --mode=debug
+
+This downloads to `~/.ue-toolchain` by default. xmake automatically detects the
+toolchain at this location, so no extra flags are needed:
+
+```bash
+xmake config -y -m debug
 xmake build
 ```
 
-Note that the command above to set the build variant to debug is optional. Tests
-are only built in debug.The `xmake` flags `-vD` can be useful to diagnose
-`xmake` issues.
+The toolchain can also be selected explicitly with `--toolchain=ue-clang`, and
+the SDK location can be overridden with `--sdk=<path>` or the
+`UE_TOOLCHAIN_DIR` environment variable.
 
 ## Building on Mac