MT#63051 debian: Fix make variable usage

These variables used to be shell variables passed to the make
invocations in various rules. But those got refactored into make
variables. The problem is that in make the equal sign does not bind
according to surrounding lack of spaces, and the first equal sign
will mark an assignment for the rest of the string including subsequent
equal signs.

The other problem is with the CPPFLAGS assignment where it will assign
append the value and then subsequent conditional assignments will not
take place. Switch to use DEB_CPPFLAGS_APPEND, which was designed
precisely for this purpose.

Fixes: commit 9b5df2495e
Change-Id: Ice82295903729fed3f0fb11f750f3f2fdde2e09f
(cherry picked from commit b7494cf7ff)

debian/rules

@@ -9,16 +9,16 @@ EXCLUDED_MODULES=conf_auth examples/db_announce fast_ack gateway ivr mailbox mp3
 EXCLUDED_DSM_MODULES=mod_aws mod_py
 EXCLUDED_DSM_PY_MODULES=mod_aws mod_py
 
-CPPFLAGS += -DHAVE_XMLRPCPP_SSL
-
-export USE_SPANDSP=yes LONG_DEBUG_MESSAGE=yes CPPFLAGS="$(CPPFLAGS)"
+export DEB_CPPFLAGS_APPEND = -DHAVE_XMLRPCPP_SSL
 
 export APP_NAME=ngcp-sems
 export SYSTEM_SAMPLECLOCK_RATE=48000LL
+export LONG_DEBUG_MESSAGE=yes
 export USE_THREADPOOL=yes
-
+export USE_SPANDSP=yes
 export PYTHON_VERSION=3
 
+
 # Enable parallel builds for overrides.
 NUMJOBS = 1
 ifneq (,$(filter parallel=%,$(DEB_BUILD_OPTIONS)))