use dpkg-buildflags if available for CXXFLAGS

debian/changelog

@@ -13,6 +13,7 @@ apt (0.7.26~exp5) UNRELEASED; urgency=low
   * debian/rules:
     - remove targets referring to CVS or arch as they are useless
     - use $(CURDIR) instead of $(pwd)
+    - use dpkg-buildflags if available for CXXFLAGS
   * README.arch:
     - remove the file completely as it has no use nowadays
   * apt-pkg/depcache.cc:

debian/rules

@@ -27,10 +27,14 @@ endif
 # See below
 -include build/environment.mak
 
-ifneq (,$(findstring noopt,$(DEB_BUILD_OPTIONS)))
-  export CXXFLAGS = -O0 -g -Wall
+ifneq (,$(shell which dpkg-buildflags))
+  export CXXFLAGS = $(shell dpkg-buildflags --get CXXFLAGS)
 else
-  export CXXFLAGS = -O2 -g -Wall
+  ifneq (,$(findstring noopt,$(DEB_BUILD_OPTIONS)))
+    export CXXFLAGS = -O0 -g -Wall
+  else
+    export CXXFLAGS = -O2 -g -Wall
+  endif
 endif
 
 # Default rule