configure.in: Fix quoting of AC_LANG_PROGRAM().

Autoconf 2.68 complains about that and aborts building the configure
script. *sigh*
mysql plugin: Call mysql_init() only once.

Apparently, if you call it with a structure allocated by mysql_init()
itself, it leaks memory. Thanks to Yves Mettier for pointing this out!

Fixes Github issue #274.