btrfs-progs: autoconf: do not force fortify flags

Make it default, but let the user override it.

Signed-off-by: David Sterba <dsterba@suse.cz>

Makefile.in

@@ -132,10 +132,10 @@ ifdef C
 	# don't use FORTIFY with sparse because glibc with FORTIFY can
 	# generate so many sparse errors that sparse stops parsing,
 	# which masks real errors that we want to see.
+	CFLAGS += -U_FORTIFY_SOURCE
 else
 	check = true
 	check_echo = true
-	CFLAGS += -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2
 endif
 
 %.o.d: %.c

configure.ac

@@ -8,7 +8,7 @@ LIBBTRFS_MAJOR=0
 LIBBTRFS_MINOR=1
 LIBBTRFS_PATCHLEVEL=1
 
-CFLAGS=${CFLAGS:-"-g -O1 -Wall"}
+CFLAGS=${CFLAGS:-"-g -O1 -Wall -D_FORTIFY_SOURCE=2"}
 AC_SUBST([CFLAGS])
 
 AC_PREREQ([2.60])