scripts/mk: Pass DEB_BUILD_OPTIONS to dpkg-buildflags

The build flags depend on options specified in DEB_BUILD_OPTIONS,
otherwise things like noopt do not work.

debian/changelog

@@ -43,6 +43,8 @@ dpkg (1.17.14) UNRELEASED; urgency=low
       warnings for code that can pose actual problems. Closes: #682659
     - Add a new canary feature, disabled by default, which will allow tracking
       build flags propagation. Closes: #628516
+  * Pass DEB_BUILD_OPTIONS to dpkg-buildflags in buildflags.mk, required to
+    get noopt to work at all, for example.
 
   [ Raphaël Hertzog ]
   * Explain better in deb-triggers(5) why interest/activate-noawait should be

scripts/mk/buildflags.mk

@@ -27,6 +27,7 @@ DPKG_BUILDFLAGS_EXPORT_ENVVAR += $(1)="$(value $(1))"
 endif
 endef
 
+$(eval $(call dpkg_buildflags_export_envvar,DEB_BUILD_OPTIONS))
 $(eval $(call dpkg_buildflags_export_envvar,DEB_BUILD_MAINT_OPTIONS))
 $(foreach flag,$(DPKG_BUILDFLAGS_LIST),\
   $(foreach operation,SET STRIP APPEND PREPEND,\