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-c.vcxproj

@@ -142,7 +142,6 @@ copy json_config.h.win32 json_config.h
   </ItemGroup>
   <ItemGroup>
     <ClInclude Include="arraylist.h" />
-    <ClInclude Include="compat\strdup.h" />
     <ClInclude Include="debug.h" />
     <ClInclude Include="json_inttypes.h" />
     <ClInclude Include="json_object.h" />
@@ -165,4 +164,4 @@ copy json_config.h.win32 json_config.h
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>
-</Project>
+</Project>