build: don't fseeko64()/ftello64() on musl libc

Check for musl libc in Patch.cpp and use fseeko()/ftello() in that case
instead of fseeko64()/ftello64() as well, as the 64 variants are glibc
extensions.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

src/common/Patch.cpp

@@ -15,8 +15,18 @@
 
 #ifndef __LIBRETRO__
 
+#ifdef __linux__
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include <features.h>
+#ifndef __USE_GNU
+#define __MUSL__
+#endif
+#endif
+
 #ifdef __GNUC__
-#if defined(__APPLE__) || defined(BSD) || defined(__NetBSD__)
+#if defined(__MUSL__) || defined(__APPLE__) || defined(BSD) || defined(__NetBSD__)
 typedef off_t __off64_t; /* off_t is 64 bits on BSD. */
 #define fseeko64 fseeko
 #define ftello64 ftello