Fix 'CONFIGURE_OPTIONS' refering to default_configure_options

Enable writing variables in $PHP_BUILD_ROOT/share/php-build/default_configure_options.

i.e.
```
--with-apxs2=/usr/local/apache-httpd-2.4.7/bin/apxs
--enable-mbstring
--enable-intl
--with-icu-dir=$(brew --prefix icu4c)
--with-gettext=$(brew --prefix gettext)
...
```