btrfs-progs: autoconf: cleanup compilation flags usage

btrfs-progs: autoconf: cleanup compilation flags usage

- define basic default CFLAGS in configure.ac, because:

   * autoconf default is -g -O2, but btrfs uses -g -O1

   * it's better to follow autoconf; standard way to modify
     CFLAGS is to call:  CFLAGS="foo bar" ./configure

- move all flags to one place in Makefile.in

- don't use AM_CFLAGS, the CFLAGS and STATIC_CFLAGS are enough

- don't mix objects and flags in $LIBS, it's more readable to
  add $(libs) to make rules

Signed-off-by: Karel Zak <kzak@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>