Try fixing the build on macOS and Windows

.github/workflows/makefile.yml

@@ -10,7 +10,11 @@ jobs:
   build:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os:
+          - ubuntu-latest
+          - macos-latest
+          # - windows-latest (The build is flaky there because of #1, unfortunately.)
+      fail-fast: false
     runs-on: ${{ matrix.os }}
 
     steps:
@@ -21,9 +25,18 @@ jobs:
         curl -L https://github.com/gbdev/rgbds/releases/download/v0.9.1/rgbds-0.9.1-linux-x86_64.tar.xz | tar xJC rgbds
         cd rgbds
         sudo ./install.sh
-    - name: Install RGBDS (macOS)
+    - name: Install RGBDS and Make (macOS)
       if: matrix.os == 'macos-latest'
-      run: brew install rgbds
+      run: |
+        brew install rgbds make
+        printf 'PATH=/opt/homebrew/opt/make/libexec/gnubin:%s\n' "$PATH" | tee -a "$GITHUB_ENV"
+    - name: Install RGBDS (Windows)
+      if: matrix.os == 'windows-latest'
+      shell: bash
+      run: | # RGBDS needs to go into that directory to be picked up by Make as `rgb*` instead of needing `rgb*.exe`.
+        mkdir rgbds
+        curl -LOJ https://github.com/gbdev/rgbds/releases/download/v0.9.1/rgbds-0.9.1-win64.zip
+        unzip -d /c/mingw64/bin rgbds-0.9.1-win64.zip
     - name: Checkout
       uses: actions/checkout@v4
       with:

Makefile

@@ -1,7 +1,7 @@
 .SUFFIXES:
 
 ifeq (${MAKE_VERSION},3.81)
-.NOTPARALLEL:
+$(error This version of Make is buggy and may fail to compile. Please install a more recent version of Make.)
 endif
 
 rwildcard = $(foreach d,$(wildcard $(1:=/*)),$(call rwildcard,$d,$2) $(filter $(subst *,%,$2),$d))