rrdtool plugin: fix thread-safety detection

rrdtool plugin: fix thread-safety detection

librrd 1.6.0 is now threadsafe and librrd_th is gone.

Since there doesn't seem to be a way to detect that librrd
is threadsafe, use pkg-config to check for 1.6.0 or newer instead.

The logic is now as follows:
Check for librrd >= 1.6.0 with pkg-config
If not found, look for rrd_update_r in librrd_th.
If not found, look for rrd_update in librrd and assume librrd is not
thread safe.