build: set OS_ID_LIKE only if unset

cmake MATCHES directive with the empty regex "" always match, including
non-empty strings.

Type: fix
Fixes: 534dfc1f18

Change-Id: If085b29da15a6d7fc680cebb823183fd3c7eea68
Signed-off-by: Benoît Ganne <bganne@cisco.com>

src/cmake/pack.cmake

@@ -65,7 +65,7 @@ macro(add_vpp_packaging)
   set(CPACK_${type}_PACKAGE_RELEASE 1)
 
   # Pure Debian does not set the "OS_ID_LIKE", it only sets "OS_ID"
-  if (OS_ID_LIKE MATCHES "")
+  if (NOT DEFINED OS_ID_LIKE)
     set(OS_ID_LIKE "${OS_ID}")
   endif()