Issue #15819: use standard autoconf preset output variables.

Reported by: Roumen Petrov

Makefile.pre.in

@@ -27,7 +27,8 @@ MODLIBS=        _MODLIBS_
 VERSION=	@VERSION@
 srcdir=		@srcdir@
 VPATH=		@srcdir@
-BUILDDIR=	@BUILDDIR@
+abs_srcdir=	@abs_srcdir@
+abs_builddir=	@abs_builddir@
 
 
 CC=		@CC@

configure

@@ -735,7 +735,6 @@ HGBRANCH
 HGTAG
 HGVERSION
 BASECPPFLAGS
-BUILDDIR
 target_alias
 host_alias
 build_alias
@@ -2747,10 +2746,8 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
 
-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

configure.ac

@@ -9,10 +9,8 @@ AC_PREREQ(2.65)
 
 AC_INIT(python, PYTHON_VERSION, http://bugs.python.org/)
 
-BUILDDIR="`pwd`"
-AC_SUBST(BUILDDIR)
 AC_SUBST(BASECPPFLAGS)
-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