Fix build warnings and extension version

[remicollet]

/dev/shm/BUILD/php-zstd-0.12.2/NTS/zstd.c: In function 'php_zstd_comp_write':
/dev/shm/BUILD/php-zstd-0.12.2/NTS/zstd.c:595:13: warning: unused variable 'ret' [-Wunused-variable]
  595 |     ssize_t ret = 0;
      |             ^~~
/dev/shm/BUILD/php-zstd-0.12.2/NTS/zstd.c: In function 'zstd_apc_serializer':
/dev/shm/BUILD/php-zstd-0.12.2/NTS/zstd.c:859:10: warning: pointer targets in assignment from 'char *' to 'unsigned char *' differ in signedness [-Wpointer-sign]
  859 |     *buf = (char*) emalloc(size + 1);
      |          ^

[remicollet]

Notice: another minor issue, PHP_ZSTD_EXT_VERSION have not be updated in 0.12.2 (still at 0.12.1)

[remicollet]

Last commit switches to the normalized macro name, so it will be checked during upload on pecl

See: https://github.com/php/web-pecl/blob/master/public_html/release-upload.php#L120

[kjdev]

Thanks.