zstd in programs/ can also receive a DEBUGLEVEL value

simplify tests : only set DEBUGLEVEL, not the flags directly

appveyor.yml

@@ -194,7 +194,7 @@
     - COMPILER: "gcc"
       HOST:     "mingw"
       PLATFORM: "x64"
-      SCRIPT:   "CPPFLAGS=-DDEBUGLEVEL=2 CFLAGS=-Werror make -j allzstd DEBUGLEVEL=2"
+      SCRIPT:   "CFLAGS=-Werror make -j allzstd DEBUGLEVEL=2"
     - COMPILER: "gcc"
       HOST:     "mingw"
       PLATFORM: "x86"