build: check for json header

Check for the json header during configuration.

Change-Id: Icf926d5fb96a3223af261eeaf3bcd2e02c4e4f86
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index 7369b45..d09a891 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,6 +38,11 @@
     [host-ipmid],
     [AC_MSG_ERROR(["phosphor-host-ipmid required and not found."])]
 )
+AC_CHECK_HEADER(
+    nlohmann/json.hpp,
+    [],
+    [AC_MSG_ERROR([Could not find nlohmann/json.hpp])]
+)
 
 # Checks for library functions.
 LT_INIT # Required for systemd linking