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>

backends/smt2/Makefile.inc

@@ -16,7 +16,7 @@ yosys-smtbmc-script.py: backends/smt2/smtbmc.py
 		-e "s|#!/usr/bin/env python3|#!$(PYTHON)|" < $< > $@
 
 yosys-smtbmc.exe: misc/launcher.c yosys-smtbmc-script.py
-	$(P) gcc -DGUI=0 -O -s -o $@ $<
+	$(P) $(CXX) -DGUI=0 -O -s -o $@ $<
 # Other targets
 else
 TARGETS += yosys-smtbmc