build: make out-of-tree builds bit-identical to in-tree builds

Previously the full source path was embedded inconsistently in the debug
information between in-tree/out-of-tree builds.

The 'vpath %.inc' becomes necessary for finding
libavfilter/all_channel_layouts.inc in out-of-tree builds.

The full source path is still embedded in the debug information, but
it's now independent of whether building in-tree or out-of-tree.

The biggest improvement of this patch is that gdb now always searches
for the path relative to the source directory. It still also searches
for the full path.
Previously it searched only for the full path in out-of-tree builds,
making the debug information generated by Debian's buildds rather hard
to use.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

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