Some Linux distributions (e.g., Debian) have install packages for ROOT that do not make use of the ROOTSYS environment variable. When you simply run ./configure and later make in such an environment, you will get error messages relating to missing files.

 ./configure CPPFLAGS=-I/usr/include/root/ LDFLAGS=-L/usr/lib/root CXXFLAGS=-O3

I would like to thank Artem Sabourov for pointing out this issue and its solution to me.