compat: add mempcpy()

The mempcpy() function was added in glibc 2.1.  It is quite handy, so
add an implementation for cross-platform use.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -331,6 +331,7 @@ extern int git_vsnprintf(char *str, size_t maxsize,
 #ifdef __GLIBC_PREREQ
 #if __GLIBC_PREREQ(2, 1)
 #define HAVE_STRCHRNUL
+#define HAVE_MEMPCPY
 #endif
 #endif
 
@@ -344,6 +345,14 @@ static inline char *gitstrchrnul(const char *s, int c)
 }
 #endif
 
+#ifndef HAVE_MEMPCPY
+#define mempcpy gitmempcpy
+static inline void *gitmempcpy(void *dest, const void *src, size_t n)
+{
+	return (char *)memcpy(dest, src, n) + n;
+}
+#endif
+
 extern void release_pack_memory(size_t, int);
 
 extern char *xstrdup(const char *str);