build: check for json header

Check for the json header during configuration.

Change-Id: I704c653e7fd51b756f65a90e541178cf1f19783b
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/configure.ac b/configure.ac
index 9ee45e0..8ad85f2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -34,6 +34,7 @@
 PKG_CHECK_MODULES([PHOSPHOR_DBUS_INTERFACES], [phosphor-dbus-interfaces],, [AC_MSG_ERROR([Could not find phosphor-dbus-interfaces...openbmc/phosphor-dbus-interfaces package required])])
 PKG_CHECK_MODULES([SDBUSPLUS], [sdbusplus],,\
     AC_MSG_ERROR(["Requires sdbusplus package."]))
+AC_CHECK_HEADER(nlohmann/json.hpp, [], [AC_MSG_ERROR([Could not find nlohmann/json.hpp])])
 
 AS_IF([test "x$enable_softoff" != "xno"], [
     PKG_CHECK_MODULES([SDEVENTPLUS], [sdeventplus],,\