Bump version requirements to base>=4.6, Cabal>=1.20

Cabal>=1.20 allows control over parallelism when compiling grammars from
Setup.hs and WebSetup.hs.

base>=4.6 allows conditional compilation with CPP to be eliminated from
a few modules.

base-4.6 corresponds to GHC 7.6.3, which is what you get in
Debian 8 (aka jessie, aka oldstable) from 2015.

gf.cabal

@@ -1,7 +1,7 @@
 name: gf
-version: 3.9
+version: 3.9-git
 
-cabal-version: >= 1.10
+cabal-version: >= 1.20
 build-type: Custom
 license: OtherLicense
 license-file: LICENSE
@@ -57,6 +57,7 @@ flag interrupt
 flag server
   Description: Include --server mode
   Default:     True
+
 flag network-uri
    description: Get Network.URI from the network-uri package
    default: True
@@ -68,6 +69,7 @@ flag network-uri
 flag custom-binary
   Description: Use a customised version of the binary package
   Default:     True
+  Manual:      True
 
 flag c-runtime
   Description: Include functionality from the C run-time library (which must be installed already)
@@ -75,7 +77,7 @@ flag c-runtime
 
 Library
   default-language:    Haskell2010
-  build-depends: base >= 4.5 && <5,
+  build-depends: base >= 4.6 && <5,
                  array,
                  containers,
                  bytestring,