Use more portable form for make builds as well

Author: zhassan-aws

src/Makefile

@@ -177,7 +177,7 @@ cadical-download:
 	@$(TAR) xfz $(cadical_release).tar.gz
 	@rm -Rf ../cadical
 	@mv cadical-$(cadical_release) ../cadical
-	@cd ../cadical && CXX=$(CXX) ./configure -O3 -s CXXFLAGS=-std=c++14 && make -j
+	@cd ../cadical && ./configure -O3 -s CXX=$(CXX) CXXFLAGS=-std=c++14 && make -j
 	@$(RM) $(cadical_release).tar.gz
 
 doc :