Support building with Clang on Windows

.github/workflows/ci.yml

@@ -608,6 +608,14 @@ jobs:
             cc: "cl", cxx: "cl",
             cxx_standard: 17,
           }
+
+          # Clang Windows
+          - {
+            name: "Windows Clang",
+            os: windows-2019,
+            cmake_args: "-T ClangCL"
+          }
+
     steps:
       - uses: actions/checkout@v2
 

CMakeLists.txt

@@ -67,11 +67,7 @@ if (MASTER_PROJECT AND CMAKE_BUILD_TYPE MATCHES Debug)
     endif()
   endif()
 
-  # Assumptions:
-  # Clang and GNU compilers run on Linux or Linux-like platforms.
-  # MSVC compilers run on Windows platforms.
-
-  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+  if(UNIX)
 
     set(
       LIBCXX_PREFIX_PATH
@@ -214,9 +210,9 @@ if (MASTER_PROJECT AND CMAKE_BUILD_TYPE MATCHES Debug)
 
     endif()
 
-  endif() # Clang or GNU
+  endif() # UNIX
 
-  if(CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
+  if(WIN32)
 
     add_compile_options(/W4)
     add_compile_options(/bigobj)
@@ -227,7 +223,7 @@ if (MASTER_PROJECT AND CMAKE_BUILD_TYPE MATCHES Debug)
       add_compile_options(/permissive-)
     endif()
 
-  endif() # MSVC
+  endif() # WIN32
 
   add_executable(
     self_test
@@ -378,7 +374,7 @@ if (MASTER_PROJECT AND CMAKE_BUILD_TYPE MATCHES Debug)
   endif()
 
   # Shameless hack to get target to work on Windows.
-  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+  if(UNIX)
 
     add_custom_target(
       run_self_test
@@ -400,7 +396,7 @@ if (MASTER_PROJECT AND CMAKE_BUILD_TYPE MATCHES Debug)
 
   endif()
 
-  if(CMAKE_CXX_COMPILER_ID MATCHES "Clang" OR CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+  if(UNIX)
 
     add_custom_target(
       run_coverage