cmake HAVE_MYSQL examples

JulNadeauCA/libagar CMakeChecks.cmake :1936

check_c_source_compiles("
#include <mysql.h>
#include <string.h>

int
main(int argc, char *argv[])
{
	MYSQL *my = mysql_init(NULL);
	if (my != NULL) { mysql_close(my); }
	return (0);
}
" HAVE_MYSQL)

polserver/polserver cmake/init.cmake :176

check_include_files (mysql/mysql.h HAVE_MYSQL)