Issue #15819: use standard autoconf preset output variables.

Reported by: Roumen Petrov
diff --git a/configure b/configure
index 6b4235c..7098353 100755
--- a/configure
+++ b/configure
@@ -649,7 +649,6 @@
 HGTAG
 HGVERSION
 BASECPPFLAGS
-BUILDDIR
 SVNVERSION
 ARFLAGS
 AR
@@ -5177,10 +5176,8 @@
 	SVNVERSION="echo Unversioned directory"
 fi
 
-BUILDDIR="`pwd`"
 
-
-if test "$srcdir" != "." -a "$srcdir" != "$BUILDDIR"; then
+if test "$abs_srcdir" != "$abs_builddir"; then
     # If we're building out-of-tree make sure Include (in the current dir)
     # gets picked up before its $srcdir counterpart in order for Python-ast.h
     # and graminit.h to get picked up from the correct directory.