Let CXX argument of "make" actually be used

C++ compiler was hardcoded to "g++" instead of using the CXX variable.

Patch 0005-Use-compilers-passed-to-make.patch from the Debian/Ubuntu
package.

rules.mk

@@ -6,12 +6,12 @@
 
 $(rastertoqpdl_TARGET): $(rastertoqpdl_OBJ)
 	$(call printCmd, $(cmd_link))
-	$(Q)g++ -o $@ $^ $(rastertoqpdl_CXXFLAGS) $(rastertoqpdl_LDFLAGS) \
+	$(Q)$(CXX) -o $@ $^ $(rastertoqpdl_CXXFLAGS) $(rastertoqpdl_LDFLAGS) \
 		$(rastertoqpdl_LIBS)
 
 $(pstoqpdl_TARGET): $(pstoqpdl_OBJ)
 	$(call printCmd, $(cmd_link))
-	$(Q)g++ -o $@ $^ $(pstoqpdl_CXXFLAGS) $(pstoqpdl_LDFLAGS) \
+	$(Q)$(CXX) -o $@ $^ $(pstoqpdl_CXXFLAGS) $(pstoqpdl_LDFLAGS) \
 		$(pstoqpdl_LIBS)
 
 .PHONY: install installcms