There is no explicit dependence to 'pkg-config' for configure process

Some version of supported OS have not installed 'pkg-config' by default. It lead to misunderstanding of error during step 'configure'
We get the error:

./configure --enable-agent
...
checking for architecture... netbsd (netbsd7.1)
checking size of void *... 8
./configure: 8957: Syntax error: word unexpected (expecting ")")

The problem is connected with dependence to 'pkg-config'

Expected: explicit dependency validation to 'pkg-config'