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The rule matcher uses a "rough-match" pre-filter, which was being too 
aggressive.  The case looked like this:

	rule:	f True
	expr:	case e of x { True -> f x }

Jues because x doesn't immediately look like True, we shouldn't say
"can't match", but that is exactly what ruleCantMatch was doing.

The constructor-specialisation optimisation was not dealing with the case
of 
	(letrec ... in f) a1 a2

We need to apply constructor specialisation in the letrec; previously
we were leaving it untouched on the grounds that the function part of
an application is almost always a variable.

But in fact, float-in immediately precedes SpecConstr, so we can get
these odd-looking applications.

The derived instance for Read of records wasn't quite right.
Consider
	data T = T1 T | T2 { x::Int }

The string "T1 T2 { x=2 }" should parse correctly as 
	T1 (T2 {x=2})
because of Haskell's odd precedence rules (record construction binds
even more tightly than application), but the derived Read didn't take
account of that.

drvrun020 is the regression test

After some earlier re-factoring, the code that was carefully trying
to make RULES available in a function's own RHS was plain wrong.

This commit fixes it.  Some programs should go faster!

Messages involving Template Haskell are deeply puzzling
if you don't know about TH, so it seems better to make
-fth an explicit flag.  It is no longer switched on
by -fglasgow-exts.

Somewhere along the 6.5 branch, gcc started compiling the rts such that
it triggers the stack smash handler, causing stage2 to by kill'd
immediately. This turns off the stack protector, which will do for now.

One-shot compilation was throwing away the old iface read by
checkOldIface, with the result that version numbers were never being
incremented.  Fixes the recomp001 test too.

The section in the building guide was becoming out of date, a wiki
page is much more likely to be kept fresh.

StgBool is mapped to C's int type.  GHC doesn't currently know the
size of a C int on the target arch, it's easier to use StgInt instead.
I guess nobody ever uses Bool arguments to foreign imports/exports.

Fixes #746