backends: smt2: use $(CXX) variable for compiler

backends: smt2: use $(CXX) variable for compiler

The Makefile assumes the compiler is called `gcc`, which isn't always
true.  In fact, if we're building on msys2 or msys2-64, the compiler
is called `i686-w64-mingw32-g++` or `x86_64-w64-mingw32-g++`.

Use the variable instead of hardcoding the name, to fix building on
these systems.

Signed-off-by: Sean Cross <sean@xobs.io>