Allow -Werror and similar when guarded by a manual flag
Allow -Werror and similar when guarded by a manual flag

Improve the package description check for -Werror and similar flags so
that they are allowed so long as they're not enabled by default. We
check the conditions that they're guarded by to make sure they're always
false (given the default values of the manual flags). Also extend the
error messages to explain how to use these flags with conditionals.

Peaker/cabal,Helkafen/cabal,edsko/cabal,gridaphobe/cabal,garetxe/cabal,Helkafen/cabal,edsko/cabal,plow-technologies/cabal,enolan/cabal,eric-stanley/cabal,mydaum/cabal,gridaphobe/cabal,themoritz/cabal,mightybyte/cabal,gbaz/cabal,sopvop/cabal,Peaker/cabal,trskop/cabal,martinvlk/cabal,thomie/cabal,lukexi/cabal,martinvlk/cabal,rimmington/cabal,ghc-android/cabal,ian-ross/cabal,randen/cabal,ian-ross/cabal,themoritz/cabal,rimmington/cabal,thoughtpolice/cabal,ghc-android/cabal,kolmodin/cabal,sopvop/cabal,valderman/cabal,plow-technologies/cabal,fugyk/cabal,valderman/cabal,fugyk/cabal,headprogrammingczar/cabal,kolmodin/cabal,mightybyte/cabal,Heather/cabal,gbaz/cabal,trskop/cabal,anton-dessiatov/cabal,bennofs/cabal,x-y-z/cabal,mydaum/cabal,Heather/cabal,garetxe/cabal,triplepointfive/cabal,beni55/cabal,thoughtpolice/cabal,bennofs/cabal,headprogrammingczar/cabal,sopvop/cabal,corngood/cabal,mmakowski/cabal,anton-dessiatov/cabal,x-y-z/cabal,mmakowski/cabal,enolan/cabal,triplepointfive/cabal,corngood/cabal,lukexi/cabal,eric-stanley/cabal,themoritz/cabal,thomie/cabal,beni55/cabal,randen/cabal