Exclude _mm_malloc/free definitions on Windows

Definitions for these functions are provided by malloc.h from both
MinGW-w64 and MSVC. When they are defined again by sse2neon, it causes
multiple definition errors. Furthermore, the implementation in sse2neon
relies on posix_memalign which is unavailable on Windows.

sse2neon.h

@@ -102,6 +102,13 @@
 #include <stdint.h>
 #include <stdlib.h>
 
+#if defined(_WIN32)
+/* Definitions for _mm_{malloc,free} are provided by <malloc.h>
+ * from both MinGW-w64 and MSVC.
+ */
+#define SSE2NEON_ALLOC_DEFINED
+#endif
+
 /* Architecture-specific build options */
 /* FIXME: #pragma GCC push_options is only available on GCC */
 #if defined(__GNUC__)
@@ -1773,10 +1780,12 @@ FORCE_INLINE __m128 _mm_div_ss(__m128 a, __m128 b)
 
 // Free aligned memory that was allocated with _mm_malloc.
 // https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm_free
+#if !defined(SSE2NEON_ALLOC_DEFINED)
 FORCE_INLINE void _mm_free(void *addr)
 {
     free(addr);
 }
+#endif
 
 // Macro: Get the flush zero bits from the MXCSR control and status register.
 // The flush zero may contain any of the following flags: _MM_FLUSH_ZERO_ON or
@@ -1958,6 +1967,7 @@ FORCE_INLINE __m128i _mm_loadu_si64(const void *p)
 // Allocate aligned blocks of memory.
 // https://software.intel.com/en-us/
 //         cpp-compiler-developer-guide-and-reference-allocating-and-freeing-aligned-memory-blocks
+#if !defined(SSE2NEON_ALLOC_DEFINED)
 FORCE_INLINE void *_mm_malloc(size_t size, size_t align)
 {
     void *ptr;
@@ -1969,6 +1979,7 @@ FORCE_INLINE void *_mm_malloc(size_t size, size_t align)
         return ptr;
     return NULL;
 }
+#endif
 
 // Conditionally store 8-bit integer elements from a into memory using mask
 // (elements are not stored when the highest bit is not set in the corresponding