Not sure if related - but in F27, I'm seeing this:

/usr/lib64/nagios/plugins $ ldd check_mysql
        linux-vdso.so.1 (0x00007ffd284ec000)
        libnsl.so.1 => /lib64/libnsl.so.1 (0x00007fc2e9fa8000)
        libresolv.so.2 => /lib64/libresolv.so.2 (0x00007fc2e9d8e000)
        libmysqlclient.so.18 => not found
        libz.so.1 => /lib64/libz.so.1 (0x00007fc2e9b77000)
        libm.so.6 => /lib64/libm.so.6 (0x00007fc2e9822000)
        libssl.so.1.1 => /lib64/libssl.so.1.1 (0x00007fc2e95b6000)
        libcrypto.so.1.1 => /lib64/libcrypto.so.1.1 (0x00007fc2e912f000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fc2e8f10000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007fc2e8d0c000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fc2e8927000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fc2ea3d1000)


However:
$ file /usr/lib64/mysql/libmysqlclient.so.18
/usr/lib64/mysql/libmysqlclient.so.18: symbolic link to libmariadb.so.3

$ file /usr/lib64/mysql/libmariadb.so.3
/usr/lib64/mysql/libmariadb.so.3: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=a5c17835b924d9830cc6b91764d3c3e80f650d05, stripped

Not sure why it isn't found - as:
/etc/ld.so.conf.d $ cat mariadb-connector-c-x86_64.conf
/usr/lib64/mariadb
/etc/ld.so.conf.d $ cat mariadb-x86_64.conf
/usr/lib64/mysql

Resolved with latest connector-c build.

The issue is that ldconfig can't use symlink
/usr/lib64/mariadb -> /usr/lib64/mysql
as a valid path.

The was released yesterday, when I found out, but it can take up to several days to reach your (and KOJI's) repository

For what it matters, I still have this issue in F27.

Not sure if its been fixed elsewhere and just needs building for F27?

For f27 I highly recommend trying this update:
https://bodhi.fedoraproject.org/updates/FEDORA-2017-d3f3d368a1

This doesn't seem to make a difference for me:

$ ./check_mysql
./check_mysql: error while loading shared libraries: libmysqlclient.so.18: cannot open shared object file: No such file or directory

$ rpm -qa | grep mysql
nagios-plugins-mysql-2.2.1-4git.fc26.x86_64
php-mysqlnd-7.1.11~RC1-1.fc27.x86_64

$ rpm -qa | grep mariadb
mariadb-common-10.2.9-3.fc27.x86_64
mariadb-errmsg-10.2.9-3.fc27.x86_64
mariadb-config-10.2.9-3.fc27.x86_64
mariadb-libs-10.2.9-3.fc27.x86_64
mariadb-10.2.9-3.fc27.x86_64
mariadb-server-10.2.9-3.fc27.x86_64
mariadb-connector-c-3.0.2-13.fc27.x86_64
mariadb-server-utils-10.2.9-3.fc27.x86_64

libmysqlclient.so.18 is present in /usr/lib64/mysql and is a symlink to libmariadb.so.3

where can I get the "check_mysql" binary?

I just noticed I had the f26 version installed - as I was testing with it....

I believe I tried the f26 version due to a higher revision number...

The package is "nagios-plugins-mysql" which gives a binary in /usr/lib64/nagios/plugins

The f27 package is:
nagios-plugins-mysql-2.2.1-2git.fc27.x86_64

I've installed that again - but the same issue.

Ah - this is why I was using the F26 nagios-plugins group:
https://bugzilla.redhat.com/show_bug.cgi?id=1500028

For you information, nagios-plugins-2.2.1-2git.fc28 fails to build in F28 because it was not ported to mariadb-connector-c:

gcc -DLOCALEDIR=\"/usr/share/locale\" -DHAVE_CONFIG_H -I. -I..  -I.. -I../lib -I../gl -I../intl -I/usr/include/ldap -I/include -I/include -I/usr/include/mysql -I/usr/include/mysql -I/include -DNP_VERSION='"2.2.1"' -I/usr/include/mysql -I/usr/include/mysql -O2 -g -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -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;
                        ^~~~~~~~~~

I have built a set that compiles with rawhide and f27. It does not use maria-connector-c yet in rawhide as the rpm it was getting still said it provided no files. I will fix later when that changes.

I'm not sure I understood.

The difference between f27 & Rawhide is that on:
Rawhide
  you should use mariadb-connector-c-devel for devel
  and mariadb-connector-c for libraries
f27
  you should use mariadb-devel and mariadb-libs for libraries.

--

The mariadb-libs does not exist anymore on Rawhide, since its functionality is porvided by Conc/C, as well as client devel files.

nagios-plugins-2.2.1-9.fc27 has been submitted as an update to Fedora 27. https://bodhi.fedoraproject.org/updates/FEDORA-2017-ef9d46f257

nagios-plugins-2.2.1-9.fc26 has been submitted as an update to Fedora 26. https://bodhi.fedoraproject.org/updates/FEDORA-2017-69f33af241

nagios-plugins-2.2.1-9.fc25 has been submitted as an update to Fedora 25. https://bodhi.fedoraproject.org/updates/FEDORA-2017-f6a0118ce5

nagios-plugins-2.2.1-9.fc25 has been pushed to the Fedora 25 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-f6a0118ce5

nagios-plugins-2.2.1-9.fc27 has been pushed to the Fedora 27 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-ef9d46f257

nagios-plugins-2.2.1-9.fc26 has been pushed to the Fedora 26 testing repository. If problems still persist, please make note of it in this bug report.
See https://fedoraproject.org/wiki/QA:Updates_Testing for
instructions on how to install test updates.
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2017-69f33af241

nagios-plugins-2.2.1-9.fc27 has been pushed to the Fedora 27 stable repository. If problems still persist, please make note of it in this bug report.