Unbundling of libsqlite

Hi!

While working on unbundling libsqlite, I've stumbled upon the
undocumented Windows only configuration option
--with-pdo-sqlite-external[1], which appears to build the
pdo_sqlite_external extension.  Is this even supported, or could it be
removed for master?

Another question: how can I check whether a certain function is
available in config.w32 (i.e. how to replace the autoconf macro
PHP_CHECK_LIBRARY)?

[1]
<https://github.com/php/php-src/blob/php-7.3.0RC1/ext/pdo_sqlite/config.w32#L15-L25>

-- 
Christoph M. Becker