#94 1.13.3 - Allow UnitTest++ Makefile CXX to inherit from ENV

[2bits sfbugs]

Currently in QuickFIX-1.13.3, the UnitTest++ Makefile is hardcoded to use

CXX = g++

It would help if that were instead

CXX ?= g++

so that UnitTest++ will build with the same compiler that QuickFIX does. This is important because some operating systems have more than one compiler which use incompatible CFLAGS. Inheriting the CFLAGS but not inheriting CXX can cause an error.