Redmine#6189: Store the hardcoded bootstrap file in a variable instead o...

...f using it directly to prevent old GCCs from failing due to fprintf usage as a macro

Ticket: https://dev.cfengine.com/issues/6189
To the language lawyer (that's me) the crucial fact is that the language specification says the toolchain is allowed to put string literals in read-only memory and conflate all instances of any given string literal, hence literals must always be treated as `const char*` despite the historical glitch of them having type `char*` formally.  It is, none the less, illuminating to see just how far some tool-chains are willing to go on that !