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.filters

@@ -50,9 +50,6 @@
     <ClInclude Include="arraylist.h">
       <Filter>Header Files</Filter>
     </ClInclude>
-    <ClInclude Include="compat\strdup.h">
-      <Filter>Header Files</Filter>
-    </ClInclude>
     <ClInclude Include="debug.h">
       <Filter>Header Files</Filter>
     </ClInclude>
@@ -96,4 +93,4 @@
     <None Include="README-WIN32.html" />
     <None Include="README.html" />
   </ItemGroup>
-</Project>
+</Project>