make: move top level dir to end of include search path

make: move top level dir to end of include search path

Currently the search path is

  1. source dir corresponding to input file (implicit by compiler)
  2. top level build dir
  3. top level source dir
  4. top level source include/ dir
  5. source dir corresponding to input file
  6. build dir corresponding to output file

Search item 5 is an effective no-op, since it duplicates item 1.
When srcdir == builddir, item 6 also duplicates item 1, which
causes a semantic difference between VPATH and non-VPATH builds.

Thus to ensure consistent semantics we need item 6 to be present
immediately after item 1. e.g.

  1. source dir corresponding to input file (implicit by compiler)
  2. build dir corresponding to output file
  3. top level build dir
  4. top level source dir
  5. top level source include/ dir

When srcdir == builddir, items 1 & 2 collapse into one, and items
3 & 4 collapse into one, but the overall search order is still
consistent with srcdir != builddir

A further complication is that while most of the source files
are built with a current directory of $BUILD_DIR, target dependant
files are built with a current directory of $BUILD_DIR/$TARGET.

As a result, search item 2 resolves to a different location for
target independant vs target dependant files. For example when
building 'migration/ram.o', the use of '-I$(@D)' (which expands
to '-Imigration') would not find '$BUILD_DIR/migration', but
rather '$BUILD_DIR/$TARGET/migration'.

If there are generated headers files to be used by the migration
code in '$BUILD_DIR/migration', these will not be found by the
relative include, an absolute include is needed instead. This
has not been a problem so far, since nothing has been generating
headers in sub-dirs, but the trace code will shortly be doing
that. So it is needed to list '-I$(BUILD_DIR)/$(@D)' as well as
'-I$(@D)' to ensure both directories are searched when building
target dependant code. So the search order ends up being:

  1. source dir corresponding to input file (implicit by compiler)
  2. build dir corresponding to output file (absolute)
  3. build dir corresponding to output file (relative to cwd)
  4. top level build dir
  5. top level source dir
  6. top level source include/ dir

One final complication is that the absolute '-I$(BUILD_DIR)/$(@D)'
will sometimes end up pointing to a non-existant directory if
that sub-dir does not have any target-independant files to be
built. Rather than try to dynamically filter this, a simple
'mkdir' ensures $(BUILD_DIR)/$(@D) is guaranteed to exist at
all times.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170125161417.31949-2-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>