teach build.rs to init submodules if needed

Forgetting to update the submodules is a commonly reported build
issue with an obscure error message:

```
CMake Error: The source directory "/home/USER/wezterm/target/debug/build/freetype-430fe24cb64c561c/out/zlib-src/zlib" does not appear to contain CMakeLists.txt.
```

this teaches the build.rs machinery to run the submodule update
if it looks like it is needed.

.github/workflows/package_posix.yml

@@ -20,8 +20,6 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1
-    - name: Update submodules
-      run: git submodule update --init
     - name: Install System Deps
       run: sudo ./get-deps
     - name: Install Rust

.github/workflows/package_win.yml

@@ -20,8 +20,6 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1
-    - name: Update submodules
-      run: git submodule update --init
     - name: Install Rust
       shell: cmd
       run: |

.github/workflows/posix.yml

@@ -18,8 +18,6 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1
-    - name: Update submodules
-      run: git submodule update --init
     - name: Install System Deps
       run: sudo ./get-deps
     - name: Install Rust

.github/workflows/windows.yml

@@ -18,8 +18,6 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v1
-    - name: Update submodules
-      run: git submodule update --init
     - name: Install Rust
       shell: cmd
       run: |

deps/freetype/build.rs

@@ -4,6 +4,10 @@ use std::env;
 use std::path::{Path, PathBuf};
 
 fn zlib() {
+    if !Path::new("zlib/.git").exists() {
+        git_submodule_update();
+    }
+
     // The out-of-source build for zlib unfortunately modifies some of
     // the sources, leaving the repo with a dirty status.  Let's take
     // a copy of the sources so that we don't trigger this.
@@ -49,6 +53,10 @@ fn libpath(p: &Path, name: &str) -> PathBuf {
 }
 
 fn libpng() {
+    if !Path::new("libpng/.git").exists() {
+        git_submodule_update();
+    }
+
     let mut config = Config::new("libpng");
     let dst = config.profile("Release").build();
     emit_libdirs(&dst);
@@ -60,6 +68,10 @@ fn libpng() {
 }
 
 fn freetype() {
+    if !Path::new("freetype2/.git").exists() {
+        git_submodule_update();
+    }
+
     let mut config = Config::new("freetype2");
     let dst = config
         .define("FT_WITH_PNG", "ON")
@@ -73,6 +85,12 @@ fn freetype() {
     println!("cargo:lib={}", libpath(&dst, "freetype").display());
 }
 
+fn git_submodule_update() {
+    let _ = std::process::Command::new("git")
+        .args(&["submodule", "update", "--init"])
+        .status();
+}
+
 fn main() {
     zlib();
     libpng();

deps/harfbuzz/build.rs

@@ -3,6 +3,10 @@ use std::env;
 use std::path::Path;
 
 fn harfbuzz() {
+    if !Path::new("harfbuzz/.git").exists() {
+        git_submodule_update();
+    }
+
     let mut config = Config::new("harfbuzz");
     for (key, value) in std::env::vars() {
         println!("{}: {}", key, value);
@@ -44,6 +48,12 @@ fn emit_libdirs(p: &Path) {
     }
 }
 
+fn git_submodule_update() {
+    let _ = std::process::Command::new("git")
+        .args(&["submodule", "update", "--init"])
+        .status();
+}
+
 fn main() {
     harfbuzz();
     let out_dir = env::var("OUT_DIR").unwrap();