Make build system honour user variables

--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,4 @@
-CFLAGS = -O3 -funroll-loops -Winline -DNDEBUG=1
-#CFLAGS = -O3 -funroll-loops -Winline
-LDLIBS = -lm -static
-# LDLIBS = -lm
+LDLIBS = -lm
 
 OBJ = .o
 EXE =
@@ -9,17 +6,13 @@
 RM = rm -f
 CP = cp
 
-GPP = g++
-LD = $(GPP) $(CFLAGS)
-CPP = $(GPP) -c $(CFLAGS) 
-
 all: pilercr
 
 CPPSRC = $(sort $(wildcard *.cpp))
 CPPOBJ	= $(subst .cpp,.o,$(CPPSRC))
 
-$(CPPOBJ): %.o: %.cpp
-	$(CPP) $< -o $@
+%.o: %.cpp
+	$(CXX) $(CXXFLAGS) -DNDEBUG $(CPPFLAGS) -c $< -o $@
 
 pilercr: $(CPPOBJ)
-	$(LD) -o pilercr $(CPPOBJ) $(LDLIBS)
+	$(CXX) $(LDFLAGS) $(CXXFLAGS) -o pilercr $(CPPOBJ) $(LDLIBS)