Fix build system to honour user variables.

--- a/Makefile
+++ b/Makefile
@@ -19,32 +19,31 @@
 ##############################################################################
 
 SHELL   = /bin/sh
-CC      = gcc
+CC     ?= gcc
 
-CFLAGS  += -pedantic -Wall -O3
-LFLAGS = -lm $(LDFLAGS)
+LIBS    = -lm
 
 TARGET  = prodigal
 SOURCES = $(shell echo *.c)
 HEADERS = $(shell echo *.h)
 OBJECTS = $(SOURCES:.c=.o)
 
-INSTALLDIR  = /usr/local/bin
+BINDIR = $(EPREFIX)/usr/bin
 
 all: $(TARGET)
 
 $(TARGET): $(OBJECTS)
-	$(CC) $(CFLAGS) -o $@ $^ $(LFLAGS)
+	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ $(LIBS)
 
 %.o: %.c $(HEADERS)
-	$(CC) $(CFLAGS) -c -o $@ $<
+	$(CC) -Wall -pedantic $(CFLAGS) $(CPPFLAGS) -c -o $@ $<
 
 install: $(TARGET)
-	install -d -m 0755 $(INSTALLDIR)
-	install -m 0755 $(TARGET) $(INSTALLDIR)
+	install -d -m 0755 $(DESTDIR)$(BINDIR)
+	install -m 0755 $(TARGET) $(DESTDIR)$(BINDIR)
  
 uninstall:
-	-rm $(INSTALLDIR)/$(TARGET)
+	-rm $(DESTDIR)$(BINDIR)/$(TARGET)
 
 clean:
 	-rm -f $(OBJECTS)