Fix build issue with MariaDB 10.2

[Sp1l]

As of 10.2 MariaDB no longer defines MYSQL_PORT.

[waja]

Potential fix for #1508

[sni]

looks good to me

[sni]

thanks

[waja]

Is there a chance to get that also merged into maint-2.2? ;)

[ottok]

This has also been encountered in Debian, see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=919375

i686-linux-gnu-gcc -DLOCALEDIR=\"/usr/share/locale\" -DHAVE_CONFIG_H -I. -I..  -I.. -I../lib -I../gl -I../intl -I/usr/include/ldap -I/usr/include/postgresql -I/usr/include -I/usr/include/mariadb -I/usr/include/ma
riadb/mysql -Wdate-time -D_FORTIFY_SOURCE=2 -DNP_VERSION='"2.2"' -I/usr/include/mariadb -I/usr/include/mariadb/mysql -g -O2 -fdebug-prefix-map=/build/monitoring-plugins-2.2=. -fstack-protector-strong -Wformat -We
rror=format-security -Wall -Wl,-z,now -MT check_mysql-check_mysql.o -MD -MP -MF .deps/check_mysql-check_mysql.Tpo -c -o check_mysql-check_mysql.o `test -f 'check_mysql.c' || echo './'`check_mysql.c
check_mysql.c:61:24: error: 'MYSQL_PORT' undeclared here (not in a function); did you mean 'MYSQL_STMT'?
 unsigned int db_port = MYSQL_PORT;
                        ^~~~~~~~~~
                        MYSQL_STMT
make[3]: *** [Makefile:1869: check_mysql-check_mysql.o] Error 1

If this PR was also merged on the maint-2.2 branch as suggested above, it would be fixed upstream and soon automatically in Debian as well.