$ pkg-config --cflags mysqlclient
Package openssl was not found in the pkg-config search path.
Perhaps you should add the directory containing `openssl.pc'
to the PKG_CONFIG_PATH environment variable
Package 'openssl', required by 'mysqlclient', not found

The reason of the error is that /usr/local/libdata/pkgconfig/mysqlclient.pc includes this line:
Requires.private: openssl

FreeBSD base doesn't provide a openssl.pc file, so attempts to use mysqlclient.pc always fails if there is no openssl port installed.