compat/strdup.h: move common compat check for strdup() to own file

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

json_tokener.c

@@ -31,6 +31,7 @@
 #include "json_object.h"
 #include "json_tokener.h"
 #include "json_util.h"
+#include "compat/strdup.h"
 
 #ifdef HAVE_LOCALE_H
 #include <locale.h>
@@ -41,13 +42,6 @@
 
 #define jt_hexdigit(x) (((x) <= '9') ? (x) - '0' : ((x) & 7) + 9)
 
-#if !HAVE_STRDUP && defined(_MSC_VER)
-  /* MSC has the version as _strdup */
-# define strdup _strdup
-#elif !HAVE_STRDUP
-# error You do not have strdup on your system.
-#endif /* HAVE_STRDUP */
-
 #if !HAVE_STRNCASECMP && defined(_MSC_VER)
   /* MSC has the version as _strnicmp */
 # define strncasecmp _strnicmp