diff --git a/src/libFLAC/CMakeLists.txt b/src/libFLAC/CMakeLists.txt
index 438399f6c0..3c75faa3b0 100644
--- a/src/libFLAC/CMakeLists.txt
+++ b/src/libFLAC/CMakeLists.txt
@@ -21,6 +21,16 @@ if(FLAC__CPU_X86_64 OR FLAC__CPU_IA32)
     if(WITH_AVX AND MSVC)
         set_source_files_properties(fixed_intrin_avx2.c lpc_intrin_avx2.c stream_encoder_intrin_avx2.c lpc_intrin_fma.c PROPERTIES COMPILE_FLAGS /arch:AVX2)
     endif()
+    if(WITH_AVX AND WIN32 AND CMAKE_C_COMPILER_ID MATCHES "Clang")
+        cmake_policy(PUSH)
+        cmake_policy(SET CMP0075 NEW)
+        check_include_file("unistd.h" HAVE_UNISTD_H)
+        if(NOT HAVE_UNISTD_H)
+            # Clang on Windows using MSVC headers
+            set_source_files_properties(fixed_intrin_avx2.c lpc_intrin_avx2.c stream_encoder_intrin_avx2.c lpc_intrin_fma.c PROPERTIES COMPILE_FLAGS -mavx2)
+        endif()
+        cmake_policy(POP)
+    endif()
 else()
     check_cpu_arch_arm64(FLAC__CPU_ARM64)
     if(FLAC__CPU_ARM64)
diff --git a/src/share/getopt/getopt.c b/src/share/getopt/getopt.c
index 509c91dd8f..4691d8f7a1 100644
--- a/src/share/getopt/getopt.c
+++ b/src/share/getopt/getopt.c
@@ -221,7 +221,7 @@ static char *posixly_correct;
 /* Avoid depending on library functions or files
    whose names are inconsistent.  */
 
-#ifndef getenv
+#if !defined(getenv) && !(defined(__clang__) && defined(_MSC_VER))
 extern char *getenv (const char * name);
 #endif