[3PP/MacOS] Fix ffmpeg and sha256 build

ffmpeg: macOS uses "_" symbol prefix (Mach-O), has no <malloc.h>,
and doesn't support .data.rel.ro sections. Add __APPLE__ guards
in ffmpeg-xenia/config.h.

sha256: macOS provides endian macros in <machine/endian.h> instead
of <endian.h>.

third_party/crypto/sha256.cpp

@@ -9,8 +9,12 @@
 
 // big endian architectures need #define __BYTE_ORDER __BIG_ENDIAN
 #ifndef _MSC_VER
+#ifdef __APPLE__
+#include <machine/endian.h>
+#else
 #include <endian.h>
 #endif
+#endif
 
 namespace sha256 {
 

third_party/ffmpeg-xenia/config.h

@@ -11,8 +11,13 @@
 #define FFMPEG_LICENSE "LGPL version 2.1 or later"
 #define CONFIG_THIS_YEAR 2021
 #define av_restrict restrict
+#ifdef __APPLE__
+#define EXTERN_PREFIX "_"
+#define EXTERN_ASM _
+#else
 #define EXTERN_PREFIX ""
 #define EXTERN_ASM
+#endif
 #define BUILDSUF ""
 #define HAVE_MMX2 HAVE_MMXEXT
 #define SWS_MAX_FILTER_SIZE 256
@@ -292,7 +297,11 @@
   #define HAVE_TERMIOS_H 1
   #define HAVE_SEM_TIMEDWAIT 1
   #define HAVE_SYNC_VAL_COMPARE_AND_SWAP 1
+#ifdef __APPLE__
+  #define HAVE_SECTION_DATA_REL_RO 0
+#else
   #define HAVE_SECTION_DATA_REL_RO 1
+#endif
   #define HAVE_CLOCK_GETTIME 1
   #define HAVE_FCNTL 1
   #define HAVE_GETADDRINFO 1
@@ -351,7 +360,11 @@
 #define HAVE_THREADS 1
 #define HAVE_ACCESS 1
 #define HAVE_ISATTY 1
+#ifdef __APPLE__
+#define HAVE_MALLOC_H 0
+#else
 #define HAVE_MALLOC_H 1
+#endif
 #define HAVE_PRAGMA_DEPRECATED 1
 
 /* Math functions */