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

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

json-c.vcproj

@@ -131,6 +131,9 @@ copy json_config.h.win32 json_config.h"/>
 			<File
 				RelativePath=".\bits.h">
 			</File>
+			<File
+				RelativePath=".\compat\strdup.h">
+			</File>
 			<File
 				RelativePath=".\debug.h">
 			</File>