Added error message for doxygen not available to configure.ac.
Added error message for doxygen not available to configure.ac.

JoaoRodrigues/freesasa,JoaoRodrigues/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,mittinatten/freesasa,mittinatten/freesasa