Add an option to disable expat

This patch replaces the existing --with-system-expat option with a
--with-expat={system,builtin,none} option, which allows to tell Python
whether we want to use the system expat (already installed), the expat
builtin the Python sources, or no expat at all (which disables the
installation of XML modules).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>

---
 Makefile.pre.in |    6 +++++-
 configure.ac    |   18 +++++++++++++-----
 setup.py        |    2 +-
 3 files changed, 19 insertions(+), 7 deletions(-)

Index: b/Makefile.pre.in
===================================================================
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -1134,7 +1134,7 @@
 		logging csv wsgiref urllib \
 		ctypes ctypes/macholib \
 		idlelib idlelib/Icons \
-		distutils distutils/command $(XMLLIBSUBDIRS) \
+		distutils distutils/command \
 		importlib \
 		turtledemo \
 		multiprocessing multiprocessing/dummy \
@@ -1205,6 +1205,10 @@
 LIBSUBDIRS += curses
 endif
 
+ifeq (@EXPAT@,yes)
+LIBSUBDIRS += $(XMLLIBSUBDIRS)
+endif
+
 ifeq (@TEST_MODULES@,yes)
 LIBSUBDIRS += $(TESTSUBDIRS)
 endif
Index: b/configure.ac
===================================================================
--- a/configure.ac
+++ b/configure.ac
@@ -2363,13 +2363,21 @@
 AC_SUBST(DISABLED_EXTENSIONS)
 
 # Check for use of the system expat library
-AC_MSG_CHECKING(for --with-system-expat)
-AC_ARG_WITH(system_expat,
-            AS_HELP_STRING([--with-system-expat], [build pyexpat module using an installed expat library]),
+AC_MSG_CHECKING(for --with-expat)
+AC_ARG_WITH(expat,
+            AS_HELP_STRING([--with-expat], [select which expat version to use: system, builtin, none]),
             [],
-            [with_system_expat="no"])
+            [with_expat="builtin"])
 
-AC_MSG_RESULT($with_system_expat)
+AC_MSG_RESULT($with_expat)
+
+if test "$with_expat" != "none"; then
+   EXPAT=yes
+else
+   DISABLED_EXTENSIONS="${DISABLED_EXTENSIONS} pyexpat"
+   EXPAT=no
+fi
+AC_SUBST(EXPAT)
 
 # Check for use of the system libffi library
 AC_MSG_CHECKING(for --with-system-ffi)
Index: b/setup.py
===================================================================
--- a/setup.py
+++ b/setup.py
@@ -1396,7 +1396,7 @@
         #
         # More information on Expat can be found at www.libexpat.org.
         #
-        if '--with-system-expat' in sysconfig.get_config_var("CONFIG_ARGS"):
+        if '--with-expat=system' in sysconfig.get_config_var("CONFIG_ARGS"):
             expat_inc = []
             define_macros = []
             expat_lib = ['expat']