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@
 CXX=		@CXX@

configure

@@ -649,7 +649,6 @@ HGBRANCH
 HGTAG
 HGVERSION
 BASECPPFLAGS
-BUILDDIR
 SVNVERSION
 ARFLAGS
 AR
@@ -5177,10 +5176,8 @@ else
 	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.

configure.ac

@@ -854,10 +854,8 @@ else
 	SVNVERSION="echo Unversioned directory"
 fi
 
-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 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.