CI: use libc++ for Clang 3.5 to 4.0

The fixes the error: "__STRICT_ANSI__ seems to have been undefined; this is not supported" with Clang 3.9

.github/workflows/ci.yml

@@ -73,11 +73,17 @@ jobs:
       matrix:
         include:
           - clang: "3.5"
+            cxxflags: "-stdlib=libc++"
           - clang: "3.6"
+            cxxflags: "-stdlib=libc++"
           - clang: "3.7"
+            cxxflags: "-stdlib=libc++"
           - clang: "3.8"
+            cxxflags: "-stdlib=libc++"
           - clang: "3.9"
+            cxxflags: "-stdlib=libc++"
           - clang: "4.0"
+            cxxflags: "-stdlib=libc++"
           - clang: "5.0"
           - clang: "6.0"
           - clang: "7"
@@ -105,7 +111,9 @@ jobs:
         env:
           CC: clang-${{ matrix.clang }}
           CXX: clang++-${{ matrix.clang }}
-          CXXFLAGS: ${{ matrix.cxxflags }}
+          CXXFLAGS: >-
+            ${{ matrix.cxxflags }}
+            ${{ contains(matrix.cxxflags, 'libc++') && '-I/usr/lib/llvm-10/include/c++/v1/' || '' }}
       - name: Build
         run: cmake --build .
       - name: Test