Fix picojson header file issue

Use the built-in picojson git submodule instead of system-wide picojson

Author: chenzhiwei

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "3rdparty/picojson"]
+	path = 3rdparty/picojson
+	url = https://github.com/kazuho/picojson

3rdparty/picojson

@@ -60,6 +60,9 @@ else
   fi
 fi
 
+basedir=$(cd $(dirname $0) && pwd)
+MESOS_CPPFLAGS="${MESOS_CPPFLAGS} -I${basedir}/3rdparty/picojson"
+
 AC_SUBST(MESOS_CPPFLAGS)
 AC_SUBST(MESOS_LDFLAGS)
 
@@ -155,10 +158,6 @@ AC_CHECK_HEADER([google/protobuf/message.h],
                     [AC_MSG_ERROR([google protobuf is not installed.])])],
                 [AC_MSG_ERROR([google protobuf is not installed.])])
 
-AC_CHECK_HEADER([picojson.h],
-                [],
-                [AC_MSG_ERROR([picojson is not installed.])])
-
 AC_CHECK_HEADERS([boost/lexical_cast.hpp boost/functional/hash.hpp],
                 [],
                 [AC_MSG_ERROR([boost is not installed.])])