autoconf: Variables listed in VPATH

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1 12.18.1 Variables listed in `VPATH'
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1 Do not set `VPATH' to the value of another variable, for example `VPATH
1 = $(srcdir)', because some ancient versions of `make' do not do
1 variable substitutions on the value of `VPATH'.  For example, use this
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1      srcdir = @srcdir@
1      VPATH = @srcdir@
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1 rather than `VPATH = $(srcdir)'.  Note that with GNU Automake, there is
1 no need to set this yourself.
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