Light speed invariance proof is circular!

Kenseto pointed out (on another thread) re the alleged invariance of
light speed:

This is a postulate....never been proven. The speed of light is a
defined constant. To do that physicists define the length of a meter
as 1/299,792,458 light-second. As you can see with this definition of
a meter length the speed of light is always 299,792,458 meters/
second.
This is a circular definition....the speed of light is defined by the
speed of light!!!!!
A new theory of relativity called IRT is invented. IRT resolvces all
the paradoxes encountered by SR due to the circular definition for
the
speed of light. A paper on IRT is available in the following link:
http://www.modelmechanics,org/2011irt.dtg.pdf

Of course, why did I not see this myself??!! Now the whole metric
system is diddled to prop up this fershlugginer bunch of Satanic
lies!!!!

But there's a way out! YES INDEEDY. You see, they might have redefined
the meter according to the "invariant" speed of light, but we can get
back at the Beast worshippers by making sure we never use the metric
system, invented by those cheese-eating surrender monkeys the French.
We can use the good ole US OF A feet and miles system. THAT will put a
stop to that little gallop, my oath it will, yes indeedy. So repeat
after me:

I wanna be wid Jesus at the love feast,
So down wid the U.N.
and let's knacker the Beast.

Myles (the scales have fallen from my eyes) Paulsen