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

This reverts commit aaba8c1080.

This commit breaks builddir != srcdir build, but, more importantly, it
also adds a dependency on a header, "config.h", which is not installed
and it's supposed to be private — since it's generated at configuration
time and it's not meant to be used by projects compiling against a
library.

json_tokener.c

@@ -31,7 +31,6 @@
 #include "json_object.h"
 #include "json_tokener.h"
 #include "json_util.h"
-#include "compat/strdup.h"
 
 #ifdef HAVE_LOCALE_H
 #include <locale.h>
@@ -42,6 +41,13 @@
 
 #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