Fix Windows build with clang-cl

clang-cl is LLVM's MSVC-compatible compiler frontend for Windows ABI.
If clang-cl is in use, `CMAKE_C_COMPILER_ID` is `Clang` even though
it doesn't take Unix-like command line options but MSVC-like options.

`if(MSVC)` is the correct predicate to check if we should pass MSVC-ish
command line options.

c/CMakeLists.txt

@@ -14,7 +14,7 @@ set(BLAKE3_AMD64_NAMES amd64 AMD64 x86_64)
 set(BLAKE3_X86_NAMES i686 x86 X86)
 set(BLAKE3_ARMv8_NAMES aarch64 AArch64 arm64 ARM64 armv8 armv8a)
 # default SIMD compiler flag configuration (can be overriden by toolchains or CLI)
-if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
+if(MSVC)
   set(BLAKE3_CFLAGS_SSE2 "/arch:SSE2" CACHE STRING "the compiler flags to enable SSE2")
   # MSVC has no dedicated sse4.1 flag (see https://learn.microsoft.com/en-us/cpp/build/reference/arch-x86?view=msvc-170)
   set(BLAKE3_CFLAGS_SSE4.1 "/arch:AVX" CACHE STRING "the compiler flags to enable SSE4.1")
@@ -77,7 +77,7 @@ endmacro()
 if(CMAKE_SYSTEM_PROCESSOR IN_LIST BLAKE3_AMD64_NAMES OR BLAKE3_USE_AMD64_ASM)
   set(BLAKE3_SIMD_AMD64_ASM ON)
 
-  if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
+  if(MSVC)
     enable_language(ASM_MASM)
     target_sources(blake3 PRIVATE
       blake3_avx2_x86-64_windows_msvc.asm