Kludgily fixed a travis build problem.  These makefiles should probably be revised to use autotools, CMake, or at least to comply with best practices such as using CXX for C++ builds, CC for C builds, and not overriding .o targets, etc...  Most annoyingly, this makefile used to override the user's preference for CXX from the environment, so the only way to set the compiler was to edit the makefile rather than using an environment variable.  Now the env var CXX will set the compiler to be used if defined.