#3647 configure fails complainign about boost wher there is no C++ compiler

[Philipp Klaus Krause]

An a system without a C++ compiler, ./configure fails complaining about missing boost, even when boost is installed:

checking for boost/graph/adjacency_list.hpp... no
configure: error: boost library not found (boost/graph/adjacency_list.hpp).

This error message is misleading. It should complain about a missing C++ compiler instead.