Fixed darwin build (#178)

* RSIZE_MAX is already defined by system on darwin

* MADV_HUGEPAGE is not available on darwin

* Fixed linker error 'Undefined symbol _get_ext_lib_export_apis' on darwin when building iwasm dylib

core/shared/platform/darwin/bh_definition.c

@@ -5,6 +5,10 @@
 
 #include "bh_platform.h"
 
+#ifdef RSIZE_MAX
+#undef RSIZE_MAX
+#endif
+
 #define RSIZE_MAX 0x7FFFFFFF
 
 int b_memcpy_s(void * s1, unsigned int s1max, const void * s2, unsigned int n)

core/shared/platform/darwin/bh_platform.c

@@ -137,12 +137,14 @@ bh_mmap(void *hint, uint32 size, int prot, int flags)
         request_size -= size;
     }
 
+#ifndef __APPLE__
     if (size >= 2 * 1024 * 1024) {
         /* Try to use huge page to improve performance */
         if (!madvise(addr, size, MADV_HUGEPAGE))
             /* make huge page become effective */
             memset(addr, 0, size);
     }
+#endif
 
     return addr_aligned;
 }