Issue #15819: use standard autoconf preset output variables.

Reported by: Roumen Petrov
diff --git a/configure b/configure
index a619bb3..6e1825f 100755
--- a/configure
+++ b/configure
@@ -715,7 +715,6 @@
 HGTAG
 HGVERSION
 BASECPPFLAGS
-BUILDDIR
 target_alias
 host_alias
 build_alias
@@ -2721,10 +2720,8 @@
 
 
 
-BUILDDIR="`pwd`"
 
-
-if test "$srcdir" != "." -a "$srcdir" != "$BUILDDIR"; then
+if test "$abs_srcdir" != "$abs_builddir"; then
     # If we're building out-of-tree, we need to make sure the following
     # resources get picked up before their $srcdir counterparts.
     #   Objects/ -> typeslots.inc