Add support in Python build system to specify host pgen

Python needs a "pgen" program to build itself. Unfortunately, the
Python build system assumes that it can use the pgen program it has
just built to build itself. Obviously, this cannot work in
cross-compilation mode since the pgen program have been built for the
target.

Therefore, this patch adds support in the Python build system for the
new PGEN_FOR_BUILD variable, so that we can point Python ./configure
script to the pgen program that have been previously built for the
host.

Patch ported to python2.7 by Maxime Ripard <ripard@archos.com>, and
later significantly reworked by Thomas Petazzoni
<thomas.petazzoni@free-electrons.com>, with some inspiration taken
from the Python patches of the PTXdist project, and then ported to
python3.3 by Maxime Ripard <maxime.ripard@free-electrons.com>

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
---
 Makefile.pre.in |    5 +++--
 configure.ac    |    5 +++++
 2 files changed, 8 insertions(+), 2 deletions(-)

Index: Python-3.3.0/Makefile.pre.in
===================================================================
--- Python-3.3.0.orig/Makefile.pre.in
+++ Python-3.3.0/Makefile.pre.in
@@ -239,6 +239,7 @@
 ##########################################################################
 # Parser
 PGEN=		Parser/pgen$(EXE)
+PGEN_FOR_BUILD=@PGEN_FOR_BUILD@
 
 PSRCS=		\
 		Parser/acceler.c \
@@ -639,8 +640,8 @@
 
 $(GRAMMAR_H): $(GRAMMAR_INPUT) $(PGENSRCS)
 		@$(MKDIR_P) Include
-		$(MAKE) $(PGEN)
-		$(PGEN) $(GRAMMAR_INPUT) $(GRAMMAR_H) $(GRAMMAR_C)
+		$(MAKE) $(PGEN_FOR_BUILD)
+		$(PGEN_FOR_BUILD) $(GRAMMAR_INPUT) $(GRAMMAR_H) $(GRAMMAR_C)
 $(GRAMMAR_C): $(GRAMMAR_H) $(GRAMMAR_INPUT) $(PGENSRCS)
 		$(MAKE) $(GRAMMAR_H)
 		touch $(GRAMMAR_C)
Index: Python-3.3.0/configure.ac
===================================================================
--- Python-3.3.0.orig/configure.ac
+++ Python-3.3.0/configure.ac
@@ -51,10 +51,15 @@
         AC_MSG_RESULT($interp)
 	PYTHON_FOR_BUILD="_PYTHON_PROJECT_BASE=$srcdir"' _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib:$(srcdir)/Lib/plat-$(MACHDEP) '$interp
     fi
+    AC_MSG_CHECKING(pgen for build)
+    PGEN_FOR_BUILD="${PGEN_FOR_BUILD}"
+    AC_MSG_RESULT($PGEN_FOR_BUILD)
 else
     PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E'
+    PGEN_FOR_BUILD='./$(PGEN)'
 fi
 AC_SUBST(PYTHON_FOR_BUILD)
+AC_SUBST(PGEN_FOR_BUILD)
 
 dnl Ensure that if prefix is specified, it does not end in a slash. If
 dnl it does, we get path names containing '//' which is both ugly and