Set -fdebug-prefix-map=... in compiler options for kernel and userland

For userland we already got this through dpkg-buildflags, but it uses
the current directory rather than the source package's top directory
as the default 'old' path.  Set DEB_BUILD_PATH to fix this.

debian/changelog

@@ -6,6 +6,7 @@ linux (4.8~rc5-1~exp2) UNRELEASED; urgency=medium
   [ Ben Hutchings ]
   * Fix some new reproducibility issues:
     - linux-source: Exclude Python bytecode generated when running Sphinx
+    - Set -fdebug-prefix-map=... in compiler options for kernel and userland
 
  -- Ben Hutchings <ben@decadent.org.uk>  Fri, 16 Sep 2016 16:39:53 +0100
 

debian/rules.real

@@ -29,6 +29,7 @@ export CROSS_COMPILE
 export PYTHONPATH = $(CURDIR)/debian/lib/python
 export DH_OPTIONS
 export DEB_HOST_ARCH DEB_HOST_GNU_TYPE DEB_BUILD_ARCH
+export DEB_BUILD_PATH = $(CURDIR)  # for dpkg-buildflags
 export LC_ALL = C.UTF-8
 
 include debian/rules.defs
@@ -38,7 +39,7 @@ stamp = [ -d $(dir $@) ] || mkdir $(dir $@); touch $@
 setup_env := env -u ABINAME -u ARCH -u FEATURESET -u FLAVOUR -u VERSION -u LOCALVERSION
 setup_env += DISTRIBUTION_OFFICIAL_BUILD=1 DISTRIBUTOR="$(DISTRIBUTOR)" DISTRIBUTION_VERSION="$(SOURCEVERSION)" KBUILD_BUILD_TIMESTAMP="$(SOURCE_DATE)" KBUILD_BUILD_VERSION_TIMESTAMP="$(DISTRIBUTOR) $(SOURCEVERSION) ($(SOURCE_DATE_UTC_ISO))" KBUILD_BUILD_USER="$(word 1,$(subst @, ,$(MAINTAINER)))" KBUILD_BUILD_HOST="$(word 2,$(subst @, ,$(MAINTAINER)))"
 
-MAKE_CLEAN = $(setup_env) $(MAKE)
+MAKE_CLEAN = $(setup_env) $(MAKE) CFLAGS_KERNEL=-fdebug-prefix-map=$(CURDIR)=. CFLAGS_MODULE=-fdebug-prefix-map=$(CURDIR)=.
 MAKE_SELF := $(MAKE) -f debian/rules.real $(MAKEOVERRIDES)
 MAKEOVERRIDES =