Description:
------------
PHP cannot be configured to use shared mysqli module when mysqlnd is used.
mysql_config is located in /usr/bin.
"--with-mysqli=mysqlnd" works fine.
"--with-mysql=mysqlnd,shared" also works fine.

MySQL version is 5.1.41 (5.1.41-3~bpo50+1).

Reproduce code:
---------------
./configure --prefix=/usr --sysconfdir=/etc --cache-file=./config.cache --with-config-file-path=/etc/php --with-config-file-scan-dir=/etc/php/extensions --with-apxs2=/usr/bin/apxs2 --enable-bcmath=shared --with-bz2=shared --enable-calendar=shared --with-curl=shared --enable-exif=shared --enable-ftp=shared --with-gd --with-gettext=shared --with-gmp --without-imap --with-kerberos --with-ldap=shared --enable-mbstring=shared --with-mcrypt=shared --with-mhash=shared --without-mssql --with-mysql=mysqlnd,shared --with-mysqli=mysqlnd,shared --enable-mysqlnd-threading --with-openssl --enable-pcntl=shared --without-pgsql --with-pspell=shared --with-snmp=shared --with-sqlite=shared --enable-sqlite-utf8 --enable-soap=shared --enable-sockets=shared --without-sybase-ct --enable-sysvmsg=shared --enable-sysvsem=shared --enable-sysvshm=shared --with-tidy=shared --without-unixODBC --with-xmlrpc=shared --with-xsl=shared --enable-zip=shared --with-zlib=shared --enable-debug --disable-cgi --enable-cli --with-pear --enable-zend-multibyte --disable-short-tags

Expected result:
----------------
Successful configuration.

Actual result:
--------------
...
checking for MySQL support... yes
checking for specified location of the MySQL UNIX socket... no
checking for MySQL UNIX socket location... no
checking for mysql_close in -lmysqlclient... yes
checking for MySQLi support... yes
checking whether to enable embedded MySQLi support... no
mysql_config not found
configure: error: Please reinstall the mysql distribution