Respect FLAGS and fix as-needed issues

https://bugs.gentoo.org/show_bug.cgi?id=295199

--- Makefile
+++ Makefile
@@ -1,6 +1,5 @@
-CFLAGS := ${CFLAGS} -Wall -O3 -finline-functions -funroll-loops 
 LIBS := -lm
 
 all:
-	${CC} ${CFLAGS} ${LIBS} -o FastTree FastTree.c
-	${CC} ${CFLAGS} ${LIBS} -o FastTreeUPGMA FastTreeUPGMA.c
+	${CC} ${CFLAGS} ${LDFLAGS} -o FastTree FastTree.c ${LIBS}
+	${CC} ${CFLAGS} ${LDFLAGS} -o FastTreeUPGMA FastTreeUPGMA.c ${LIBS}