ghc: support building with integer-simple and provide package sets for it

If the flag enableIntegerSimple is true GHC will be build with the GPL-free but
slower integer-simple library instead of the faster but GPLed integer-gmp
library.

The attribute `pkgs.haskell.compiler.integer-simple."${ghcVersion}"` provides a
GHC compiler build with `integer-simple`.

Similarly, the attribute `pkgs.haskell.packages.integer-simple."${ghcVersion}"`
provides a package set supporting `integer-simple`.

Closes https://github.com/NixOS/nixpkgs/pull/22121.
Closes https://github.com/NixOS/nixpkgs/issues/5493.

doc/languages-frameworks/haskell.md

@@ -793,6 +793,64 @@ It's important to realize, however, that most system libraries in Nix are built
 as shared libraries only, i.e. there is just no static library available that
 Cabal could link!
 
+### Building GHC with integer-simple
+
+By default GHC implements the Integer type using the
+[GNU Multiple Precision Arithmetic (GMP) library](https://gmplib.org/).
+The implementation can be found in the
+[integer-gmp](http://hackage.haskell.org/package/integer-gmp) package.
+
+A potential problem with this is that GMP is licensed under the
+[​GNU Lesser General Public License (LGPL)](http://www.gnu.org/copyleft/lesser.html),
+a kind of "copyleft" license. According to the terms of the LGPL, paragraph 5,
+you may distribute a program that is designed to be compiled and dynamically
+linked with the library under the terms of your choice (i.e., commercially) but
+if your program incorporates portions of the library, if it is linked
+statically, then your program is a "derivative"--a "work based on the
+library"--and according to paragraph 2, section c, you "must cause the whole of
+the work to be licensed" under the terms of the LGPL (including for free).
+
+The LGPL licensing for GMP is a problem for the overall licensing of binary
+programs compiled with GHC because most distributions (and builds) of GHC use
+static libraries. (Dynamic libraries are currently distributed only for OS X.)
+The LGPL licensing situation may be worse: even though
+​[The Glasgow Haskell Compiler License](https://www.haskell.org/ghc/license)
+is essentially a "free software" license (BSD3), according to
+paragraph 2 of the LGPL, GHC must be distributed under the terms of the LGPL!
+
+To work around these problems GHC can be build with a slower but LGPL-free
+alternative implemention for Integer called
+[integer-simple](http://hackage.haskell.org/package/integer-simple).
+
+To get a GHC compiler build with `integer-simple` instead of `integer-gmp` use
+the attribute: `pkgs.haskell.compiler.integer-simple."${ghcVersion}"`.
+For example:
+
+    $ nix-build -E '(import <nixpkgs> {}).pkgs.haskell.compiler.integer-simple.ghc802'
+    ...
+    $ result/bin/ghc-pkg list | grep integer
+        integer-simple-0.1.1.1
+
+The following command displays the complete list of GHC compilers build with `integer-simple`:
+
+    $ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler.integer-simple
+    haskell.compiler.integer-simple.ghc7102  ghc-7.10.2
+    haskell.compiler.integer-simple.ghc7103  ghc-7.10.3
+    haskell.compiler.integer-simple.ghc722   ghc-7.2.2
+    haskell.compiler.integer-simple.ghc742   ghc-7.4.2
+    haskell.compiler.integer-simple.ghc763   ghc-7.6.3
+    haskell.compiler.integer-simple.ghc783   ghc-7.8.3
+    haskell.compiler.integer-simple.ghc784   ghc-7.8.4
+    haskell.compiler.integer-simple.ghc801   ghc-8.0.1
+    haskell.compiler.integer-simple.ghc802   ghc-8.0.2
+    haskell.compiler.integer-simple.ghcHEAD  ghc-8.1.20170106
+
+To get a package set supporting `integer-simple` use the attribute:
+`pkgs.haskell.packages.integer-simple."${ghcVersion}"`. For example
+use the following to get the `scientific` package build with `integer-simple`:
+
+    $ nix-build -A pkgs.haskell.packages.integer-simple.ghc802.scientific
+
 
 ## Other resources