Autotools: Fix pdo_mysql build with mysql client library (#15450)

When using --with-pdo-mysql=DIR and for some reason DIR wouldn't have
the executable DIR/bin/mysql_config available the 3rd option was so far
to guess the mysql client library locations and pass the include
directory further. The library directory and library name to link was
missing. This fixes this case for consistency reasons. And in the future
also the pkg-config could be a reliable alternative to mysql_config in
that case. The PDO_MYSQL_INC_DIR is already processed PHP_EVAL_INCLINE
so redundant PHP_ADD_INCLUDE is removed.

Author: petk

ext/pdo_mysql/config.m4

@@ -65,8 +65,8 @@ if test "$PHP_PDO_MYSQL" != "no"; then
         AC_MSG_ERROR([Unable to find your mysql installation])
       fi
 
-      PHP_ADD_INCLUDE([$PDO_MYSQL_INC_DIR])
       PDO_MYSQL_INCLUDE=-I$PDO_MYSQL_INC_DIR
+      PDO_MYSQL_LIBS="-L$PDO_MYSQL_LIB_DIR -lmysqlclient"
     else
       AC_MSG_RESULT([not found])
       AC_MSG_ERROR([Unable to find your mysql installation])