btrfs-progs: build: note about 32bit build on 64bit host

Notes for making it work:

- "CFLAGS=-m32 LDFLAGS=-m32 ./configure ..." passes the right flags
  everywhere, otherwise just passing it by EXTRA_CFLAGS/EXTRA_LDFLAGS
  might miss some binaries or libraries

- note that passing CFLAGS/LDFLAGS to configure will override the
  default flags

- libbtrfsutil and python bindings require to pass the -m32 flags via
  EXTRA_PYTHON_CFLAGS/EXTRA_PYTHON_LDFLAGS

- all the e2fsprogs, zlib, lzo, util-linux etc need packaging changes to
  provide static libraries - usually frowned upon by distribution unless
  justified, even less so for static + 32bit versions, very niche
  use case

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

INSTALL

@@ -108,5 +108,14 @@ The list of built-ins can be obtained by
 The basic set will always contain: mkfs.btrfs, btrfs-image, btrfs-convert.
 
 
+32bit build on 64bit host
+-------------------------
+
+The combination of 32bit build on 64bit host could work but depends on the
+libraries that must provide the 32bit versions, or even 32bit static versions.
+This is fairly uncommon on contemporary distributions and building 32bit
+versions on a 32bit host is recommended.
+
+
 References:
 * https://btrfs.wiki.kernel.org