tools: work around collision of -O0 and -D_FORTIFY_SOURCE

Some systems have python-config include -D_FORTIFY_SOURCE in the
CFLAGS.  But -D_FORTIFY_SOURCE does not (currently) work with -O0, and
-O0 is enabled in debug builds (since 1166ecf781).  As a result, on
those systems, debug builds fail.

Work around this problem as follows:
 * In configure, detect -D_FORTIFY_SOURCE in $(python-config --cflags)
 * If detected, set the new autoconf substitution and make variable
   PY_NOOPT_CFLAGS to -O1.
 * In tools/Rules.mk, where we add -O0, also add PY_NOOPT_CFLAGS
   (which will override the -O0 with -O1 if required).

Overriding the -O0 is better than disabling Fortify because the
latter might have an adverse security impact.  A user who wants to
disable optimisation completely even for Python and also disable
Fortify can set the environment variable
    EXTRA_CFLAGS_XEN_TOOLS='-U_FORTIFY_SOURCE -O0'

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reported-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
CC: Jan Beulich <JBeulich@suse.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Euan Harris <euan.harris@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Don Slutz <dslutz@verizon.com>

tools/python/Makefile

@@ -4,6 +4,8 @@ include $(XEN_ROOT)/tools/Rules.mk
 .PHONY: all
 all: build
 
+PY_CFLAGS = $(CFLAGS) $(PY_NOOPT_CFLAGS) $(LDFLAGS) $(APPEND_LDFLAGS)
+
 .PHONY: build
 build: genwrap.py $(XEN_ROOT)/tools/libxl/libxl_types.idl \
 		$(XEN_ROOT)/tools/libxl/idl.py
@@ -11,11 +13,11 @@ build: genwrap.py $(XEN_ROOT)/tools/libxl/libxl_types.idl \
 		$(XEN_ROOT)/tools/libxl/libxl_types.idl \
 		xen/lowlevel/xl/_pyxl_types.h \
 		xen/lowlevel/xl/_pyxl_types.c
-	CC="$(CC)" CFLAGS="$(CFLAGS) $(LDFLAGS) $(APPEND_LDFLAGS)" $(PYTHON) setup.py build
+	CC="$(CC)" CFLAGS="$(PY_CFLAGS)" $(PYTHON) setup.py build
 
 .PHONY: install
 install:
-	CC="$(CC)" CFLAGS="$(CFLAGS) $(LDFLAGS) $(APPEND_LDFLAGS)" $(PYTHON) setup.py install \
+	CC="$(CC)" CFLAGS="$(PY_CFLAGS)" $(PYTHON) setup.py install \
 		$(PYTHON_PREFIX_ARG) --root="$(DESTDIR)" --force
 
 .PHONY: test