A root include is missing. Check that the command line executed by make has -I<where root headers are>. It could be that root-config in your case cannot be found / that you have to source thisroot.sh.
Well, make doesn’t react to -I. Those commands are for the compiler, so they need to be in the Makefile.
Also note that root-config will generate the correct flags for the compiler:
Your makefile has a root-config line, but I’m not sure its executed correctly. Check e.g. that $ROOT_BIN leads somewhere meaningful.
That’s why I said that you have to check what command line make generates for your compiler. Make usually prints the lines, unless you silenced it. You can e.g. google how to un-silence make.
BTW:
You don’t have to be “pretty sure”, you can check: