ci: use C++14+

Author: henryiii

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
     - uses: actions/checkout@v1
 
     - name: Configure
-      run: cmake -DCMAKE_VERBOSE_MAKEFILE=ON -G "MinGW Makefiles" -S . -B build
+      run: cmake -DCMAKE_VERBOSE_MAKEFILE=ON -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -S . -B build
 
     - name: Build
       run: cmake --build build -j 2