Try to support compilation with "clang-cl"

Target pragmas don't seem to work with "clang-cl", so try to
manually define the macros that are expected to be predefined.

Ugly hack; there has to be a better way to address this.

Fixes #18

Author: jedisct1

.github/workflows/clang-cl.yml

@@ -0,0 +1,31 @@
+name: Clang-CL
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+
+env:
+  BUILD_TYPE: Release
+
+jobs:
+  build:
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - uses: ilammy/msvc-dev-cmd@v1
+
+    - name: Environment
+      run: clang-cl -E -Xclang -dM nul
+
+    - name: Configure CMake
+      env:
+        CC: clang-cl
+        CMAKE_GENERATOR: Ninja
+      run: cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{env.BUILD_TYPE}}
+
+    - name: Build
+      run: cmake --build ${{github.workspace}}/build --config ${{env.BUILD_TYPE}}

src/common/common.h

@@ -43,6 +43,29 @@ static int errno;
 #            undef HAVE_VAESINTRIN_H
 #        endif
 #    endif
+
+/* target pragmas don't define these flags on clang-cl (an alternative clang driver for Windows) */
+#    if defined(__clang__) && defined(_MSC_BUILD) && defined(_MSC_VER) && \
+        (defined(_M_IX86) || defined(_M_AMD64)) && !defined(__SSE3__)
+#        undef __SSE3__ 1
+#        undef __SSSE3__ 1
+#        undef __SSE4_1__ 1
+#        undef __AVX__ 1
+#        undef __AVX2__ 1
+#        undef __AVX512F__ 1
+#        undef __AES__ 1
+#        undef __VAES__ 1
+
+#        define __SSE3__    1
+#        define __SSSE3__   1
+#        define __SSE4_1__  1
+#        define __AVX__     1
+#        define __AVX2__    1
+#        define __AVX512F__ 1
+#        define __AES__     1
+#        define __VAES__    1
+#    endif
+
 #endif
 
 #ifdef DISABLE_AVX2