Add policy::Table class

This class holds the error log policy data defined by
IBM's service people.  The raw data is stored in a JSON
file on the BMC, and loaded into the class on construction.

The policy data is a map of error messages (i.e.  the
xyz.openbmc_project.Foo.Error.Bar strings) to a list of
policy details structures.

Each details structure has:
* a search modifier - used to find the specific details
  entry for a specific error log message.
* a message - a customer facing description of the error
* an event ID - an ID defined by IBM that can be used to
                locate information about the error online.

An example of an entry of the JSON data it consumes is:
  {
    "dtls":[
      {
        "CEID":"ID 1",
        "mod":"mod 1",
        "msg":"Error 1"
      },
      {
        "CEID":"ID 2",
        "mod":"mod 2",
        "msg":"Error 2 "
      }
    ],
    "err":"xyz.openbmc_project.Error.Test1"
  }

A future commit will add the ability to find an entry.

Change-Id: I9869c15799914acf9cbc9d91ff714edb6e2512ef
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/configure.ac b/configure.ac
index 6b38795..9bbe9fb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -17,6 +17,9 @@
 PKG_CHECK_MODULES([PHOSPHOR_LOGGING], [phosphor-logging],,\
     AC_MSG_ERROR(["Requires openbmc/phosphor-logging package."]))
 
+AC_CHECK_HEADER(nlohmann/json.hpp, ,
+                [AC_MSG_ERROR([Could not find nlohmann/json.hpp... nlohmann/json package required])])
+
 # Checks for typedefs, structures, and compiler characteristics.
 AX_CXX_COMPILE_STDCXX_14([noext])
 AX_APPEND_COMPILE_FLAGS([-Wall -Werror], [CXXFLAGS])