Drop support for GHCs older than GHC 6.12

As `text` doesn't build with GHC 6.10 anymore (see below), we can drop
support for GHC < 6.12; this commit evaluates compile time conditionals to
assume that GHC>=6.12 and adds lower bounds to build-depends to help
cabal fail early.

```
Building text-0.11.3.1...
Preprocessing library text-0.11.3.1...
[28 of 41] Compiling Data.Text.IO     ( Data/Text/IO.hs, dist/build/Data/Text/IO.o )
Data/Text/IO.hs:104:34:
    Not in scope: type constructor or class `Handle__'
Data/Text/IO.hs:105:20: Not in scope: data constructor `Handle__'
Data/Text/IO.hs:106:6: Not in scope: `isEOFError'
Data/Text/IO.hs:107:15: Not in scope: `readIORef'
Data/Text/IO.hs:107:25: Not in scope: `haCharBuffer'
Data/Text/IO.hs:108:20: Not in scope: `isEmptyBuffer'
Data/Text/IO.hs:109:22: Not in scope: `T.empty'
Data/Text/IO.hs:110:22: Not in scope: `T.singleton'
Data/Text/IO.hs:111:18: Not in scope: `E.throwIO'
Data/Text/IO.hs:111:29: Not in scope: `augmentIOError'
Data/Text/IO.hs:121:14: Not in scope: `wantReadableHandle'
Data/Text/IO.hs:123:21: Not in scope: data constructor `Handle__'
Data/Text/IO.hs:124:11: Not in scope: `readIORef'
Data/Text/IO.hs:124:21: Not in scope: `haCharBuffer'
Data/Text/IO.hs:125:9: Not in scope: `readChunk'
Data/Text/IO.hs:125:26: Not in scope: `E.catch'
```