haskell: extend generic builder to recognize mis-configured Cabal 2.x builds

Cabal 1.x says:

 | Warning: This package indirectly depends on multiple versions of the same
 | package. This is highly likely to cause a compile failure.

But in version 2.x, that warning is split into two lines differently:

 | Warning:
 |     This package indirectly depends on multiple versions of the same package. This is very likely to cause a compile failure.

This commit modifies the call to "egrep" to recognize both versions by virtue
of the "-z" flag, which essentially interprets the whole configure-time output
as one long line.