btrfs-progs: build: set include path relatively

btrfs-progs: build: set include path relatively

Currently, gcc is passed the include directory with full path. As a result,
dependency files (*.o.d) also record the full path at the build time. Such
full path dependency is annoying for sharing the source between multiple
machines, containers, or anything the path differ.

And this is the same way what other program using autotools e.g. e2fsprogs
is doing:

$ grep top_builddir Makefile
top_builddir = .
CPPFLAGS = -I. -I$(top_builddir)/lib -I$(top_srcdir)/lib
BUILD_CFLAGS = -g -O2  -I. -I$(top_builddir)/lib -I$(top_srcdir)/lib -DHAVE_CONFIG_H
<snip>

Signed-off-by: Naohiro Aota <naota@elisp.net>
[ set TOPDIR=. instead of -I as discussed, does not harm linker ]
Signed-off-by: David Sterba <dsterba@suse.com>