sdl12-compat: CI: Hopefully fix CMake configure stage

SDL Commits
1 
From a34215cb80cecbd1da63da79e7305a2b99c6f055 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 1 Jun 2021 18:10:50 -0400
Subject: [PATCH] CI: Hopefully fix CMake configure stage

---
 .github/workflows/main.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 86b66e3..5df4c11 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -27,7 +27,7 @@ jobs:
     - name: Get sdl12-compat sources
       uses: actions/checkout@v2
     - name: Configure CMake
-      run: cmake -DCMAKE_C_FLAGS="-I${GITHUB_WORKSPACE}/SDL2/include" -B build ${{ matrix.platform.flags }}
+      run: cmake -DSDL2_INCLUDE_DIR="$GITHUB_WORKSPACE/SDL2/include" -B build ${{ matrix.platform.flags }}
     - name: Build
       run: cmake --build build/