Perform out-of-tree builds with Makefile

Apart from being highly desirable on its own to avoid clutter in the
root folder, this also fixes an error that occurs during cmake
out-of-tree builds if there are left-overs from a make in-tree build.

After some consideration and research, I believe that there is no
reasonable way in cmake to isolate the unwanted files from the build.
The simplest option is to perform out-of-tree builds in make as well.

In fact, the existence of in-tree generated header files during
out-of-tree builds can always lead to inconsistencies between different
translation units because we cannot reliably enfore that the out-of-tree
header is used from everywhere.

Resolves #7.

Notably, the compile-time error in #7 occured due to the fact that the
include guards of in/out-of-tree headers were different, which could
lead to both variants of a header being included, and hence redefinition
errors. However, this is not the actual issue, but rather a symptom that
we were lucky enough to observe.

Makefile

@@ -1,28 +1,33 @@
-OBJS     = main.o parser.o scanner.o citip.o
+BUILDDIR = build
+OBJS     = $(addprefix $(BUILDDIR)/,main.o parser.o scanner.o citip.o)
 CPPFLAGS = -MMD -MP
-CXXFLAGS = -std=c++11
+CXXFLAGS = -std=c++11 -I. -I$(BUILDDIR)
 
-all: Citip
+all: prepare Citip
 
 Citip: $(OBJS)
 	g++ -o $@ $^ -lglpk
 
-%.o: %.cpp
+$(BUILDDIR)/%.o: %.cpp
 	$(CXX) -o $@ -c $< $(CPPFLAGS) $(CXXFLAGS)
 
-%.o: %.cxx
+$(BUILDDIR)/%.o: $(BUILDDIR)/%.cxx
 	$(CXX) -o $@ -c $< $(CPPFLAGS) $(CXXFLAGS)
 
-parser.cxx: parser.y
-	bison -o $@ --defines=parser.hxx $<
+$(BUILDDIR)/parser.cxx: parser.y
+	bison -o $@ --defines=$(BUILDDIR)/parser.hxx $<
 
-scanner.cxx: scanner.l
-	flex -o $@ --header-file=scanner.hxx $<
+$(BUILDDIR)/scanner.cxx: scanner.l
+	flex  -o $@ --header-file=$(BUILDDIR)/scanner.hxx $<
 
-$(OBJS): scanner.cxx parser.cxx
+$(OBJS): $(BUILDDIR)/scanner.cxx $(BUILDDIR)/parser.cxx
+
+.PHONY: prepare all
+prepare:
+	@mkdir -p $(BUILDDIR)
 
 clean:
-	rm -f *.o *.cxx *.hxx *.hh *.d
+	rm -rf $(BUILDDIR)
 
 clobber: clean
 	rm -f Citip