get default CFLAGS/CXXFLAGS from configure

Since autoconf already sets up default compiler flags for us, use those.
These also come from the user's build settings.

Makefile.in

@@ -103,9 +103,11 @@ STRIP:=@STRIP@
 OBJECTS:=$(subst .cc,.o,$(SOURCE))
 TOP_DIR:=@top_srcdir@
 TOP_BUILDDIR:=@top_builddir@
-CFLAGS+=-g -Wall -O3
+CFLAGS?=@CFLAGS@
+CFLAGS+=-Wall
 CFLAGS+=@LFS_FLAGS@
-CXXFLAGS+=-g -Wall -fno-strict-aliasing -std=gnu++98
+CXXFLAGS?=@CXXFLAGS@
+CXXFLAGS+=-Wall -fno-strict-aliasing -std=gnu++98
 CXXFLAGS+=@CXXOPTIMISE_FLAG@
 CXXFLAGS+=@CXXDEBUG_FLAG@
 CXXFLAGS+=@CXX_STRERROR_FLAG@