Respect env CFLAGS and build shared objects as PIC

--- libaio/src/Makefile
+++ libaio/src/Makefile
@@ -1,7 +1,7 @@
 prefix=/usr
 
-CFLAGS=-nostdlib -nostartfiles -Wall -I. -g -O2
-SO_CFLAGS=-shared $(CFLAGS)
+CFLAGS+=-nostdlib -nostartfiles -Wall -I.
+SO_CFLAGS=-shared $(CFLAGS) -fPIC
 L_CFLAGS=$(CFLAGS)
 LINK_FLAGS=-L.