Put NDEBUG in upb, not in CFLAGS.

php/ext/google/protobuf/php-upb.h

@@ -1,3 +1,5 @@
+#define NDEBUG
+
 /* Amalgamated source file */
 #include <stdint.h>/*
 * This is where we define macros used across upb.

php/tests/compile_extension.sh

@@ -10,7 +10,7 @@ rm -f configure.in configure.ac
 php make-preload.php
 phpize
 if [ "$1" = "--release" ]; then
-  ./configure --with-php-config=$(which php-config) CFLAGS="-DNDEBUG"
+  ./configure --with-php-config=$(which php-config)
 else
   # To get debugging symbols in PHP itself, build PHP with:
   #   $ ./configure --enable-debug CFLAGS='-g -O0'