Alec Theriault authored Dec 04, 2018 and Tamar Christina committed Dec 04, 2018

Summary:
This is the latest in the 'findPtr' saga. See

  * 900c47f8
  * 561748cb

for the previous attempts. The problem with re-using the 'debug'
cabal flag for the purpose of forcing inclusion of 'findPtr' occurs
when 'debug' is one of the RTS ways, but RTS is not being compiled
with '-DDEBUG':

  * the 'debug' flag gets passed to cabal, signalling to build
    'rts' with the debug flavour, but also forcing inclusion of
    the 'findPtr'/'_findPtr' symbol

  * since '-DDEBUG' isn't enabled, that symbol doesn't show up in
    the libraries, so executable that depend on 'rts' (everything)
    will end up always requiring 'findPtr'/'_findPtr' but 'rts' won'y
    provide it!

The fix is simple: create a a new 'find-ptr' cabal-flag whose only
purpose is forcing '-Wl,-u,findPtr'/'-Wl,-u,_findPtr'. Then, enable that
flag when the RTS is being compiled with '-DDEBUG'

Test Plan: ./hadrian/build.sh...