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This makes things more robust when running with threads

Now instead of saying, e.g.
    namebase_if_compiler_lt('ghc','6.9', 'foo-6.8')
you say
    if_compiler_lt('ghc','6.9', namebase('foo-6.8'))

The rationale for these new behaviors is as follows:

We now allow existentials to be unpacked in local (but not top-level!)
ValBinds, so some pattern signatures that used to fail in the renamer
now fail in the type checker (if the type of the body mentions the
unpacked type var):
   tc141(normal)

We don't always print HsWrappers in debug mode:
   TH_exn1(normal)
   TH_exn2(normal)

Sections are parsed (and therefore printed) differently:
   rnfail019(normal)
   read023(normal)

Character numbers in pattern printing are more precise:
   tcfail067(normal)
   tcfail124(normal)
   tcfail125(normal)

#1384 = Exporting a module that isn't imported doesn't fail