Add third-party spdlog (v1.15.3) as a submodule

Also modifies Github CI config to ensure the checkout action also checks
out the submodule.

Author: jackbondpreston-arm

.github/workflows/build-linux.yml

@@ -23,6 +23,8 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
+        with:
+          submodules: true
       - name: Setup environment
         run: |
           sudo apt-get install -y libssl-dev

.github/workflows/build-windows.yml

@@ -24,6 +24,8 @@ jobs:
         uses: microsoft/setup-msbuild@v2
       - name: Checkout code
         uses: actions/checkout@v4
+        with:
+          submodules: true
       - name: Set up Python 3.10
         uses: actions/setup-python@v5
         with:

.github/workflows/super-linter.yml

@@ -24,6 +24,7 @@ jobs:
           # Full git history is needed to get a proper list of changed files
           # within `super-linter`
           fetch-depth: 0
+          submodules: true
       - name: Super-linter
         uses: super-linter/super-linter@v7.3.0 # x-release-please-version
         env:

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "third_party/spdlog"]
+	path = third_party/spdlog
+	url = https://github.com/gabime/spdlog.git

README.md

@@ -62,6 +62,23 @@ Optional dependencies are:
 
 Please refer to [docs/](docs/) for detailed documentation.
 
+## Cloning
+
+Gloo makes use of [Git submodules](https://git-scm.com/book/en/v2/Git-Tools-Submodules)
+for third-party build-time dependencies. When cloning, ensure that you
+initialize and sync submodules:
+
+``` shell
+git clone --recurse-submodules https://github.com/pytorch/gloo.git # or use SSH remote
+```
+
+Or, if you already cloned without submodules, run the following in the existing
+local repository:
+
+``` shell
+git submodule update --init --recursive
+```
+
 ## Building
 
 You can build Gloo using CMake.