Enable nlohmann-json dependency in phosphor-networkd

- This commit would enable and enforce the nlohmann-json headers
  as a dependency.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I73e784d92e3480faa7eb25c127df0c1cbebeb1c7
diff --git a/configure.ac b/configure.ac
index b6ffd6c..8dfe113 100644
--- a/configure.ac
+++ b/configure.ac
@@ -142,6 +142,14 @@
       )
 AM_CONDITIONAL([FEATURE_SYNC_MAC_FROM_INVENTORY], [test x$sync_mac = xtrue])
 
+#check for nlohmann-json header files.
+AS_IF([test "x$sync_mac" == "xtrue"],[
+AC_CHECK_HEADER(
+    nlohmann/json.hpp,
+    [],
+    [AC_MSG_ERROR([Could not find nlohmann/json.hpp])]]
+))
+
 # Create configured output.
 AC_CONFIG_FILES([Makefile test/Makefile])
 AC_CONFIG_FILES([xyz.openbmc_project.Network.service])