Add zlib to macOS dependencies.

This is to fix compilation of Pillow using Python 3.9.

Issue #162.

.github/workflows/ci.yaml

@@ -47,6 +47,10 @@ jobs:
               run: |
                   brew install bazelisk zlib
                   python -m pip install -r compiler_gym/requirements.txt -r examples/requirements.txt -r leaderboard/requirements.txt -r tests/requirements.txt
+              env:
+                  LDFLAGS: -L/usr/local/opt/zlib/lib
+                  CPPFLAGS: -I/usr/local/opt/zlib/include
+                  PKG_CONFIG_PATH: /usr/local/opt/zlib/lib/pkgconfig
               if: matrix.os == 'macos-latest'
 
             - name: Test
@@ -85,8 +89,12 @@ jobs:
 
             - name: Install dependencies (macos)
               run: |
-                  brew install bazelisk
+                  brew install bazelisk zlib
                   python -m pip install -r compiler_gym/requirements.txt -r examples/requirements.txt -r leaderboard/requirements.txt -r tests/requirements.txt
+              env:
+                  LDFLAGS: -L/usr/local/opt/zlib/lib
+                  CPPFLAGS: -I/usr/local/opt/zlib/include
+                  PKG_CONFIG_PATH: /usr/local/opt/zlib/lib/pkgconfig
               if: matrix.os == 'macos-latest'
 
             - name: Install

README.md

@@ -68,11 +68,10 @@ newer equivalents).
 If you prefer, you may build from source. This requires a modern C++ toolchain
 and bazel.
 
-On macOS you can use the system compiler and install
-[bazelisk](https://github.com/bazelbuild/bazelisk) using
+On macOS the required dependencies can be installed using
 [homebrew](https://docs.brew.sh/Installation):
 
-    $ brew install bazelisk
+    $ brew install bazelisk zlib
 
 On linux, install the required toolchain using: