A Makefile for C++ programs needs some minor modifications.
Unfortunately, for C++ there are few standards: C++ implementation files
can have a .c, .C or .cc extension, and a compiler
can recognize any combination of these extensions as source files.
Different make implementations can have different default macros
for compiler names too.
GNU Make defines CXX as g++ by default and uses
CXXFLAGS to pass flags to the C++ compiler.
Solaris Make defines CCC as CC by default and uses
CCCFLAGS to pass flags to the C++ compiler.
We should therefore never rely on default implicit rules provided by a certain
flavor of make.
Always write a %.o: %.C or %.o: %.cc pattern rule, depending on
which extension you use for your implementation files.