qapi-dir does not need an absolute path. All other build directories
are relative. When BUILD_DIR is removed, the build output looks better
(no long lines with absolute paths when everything else uses short
lines):

  GEN   qapi-generated/qga-qapi-types.c
  CC    qapi-generated/qga-qapi-types.o
  GEN   qapi-generated/qga-qapi-visit.c
  CC    qapi-generated/qga-qapi-visit.o
  GEN   qapi-generated/qga-qmp-marshal.c
  CC    qapi-generated/qga-qmp-marshal.o

Using a relative path also avoids potential problems when BUILD_DIR
includes blanks.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>