gmp: Fix build on ARM

The config.guess script tries to detect various ARM optimization flags
via /proc/cpuinfo. This is not only impure, but the detection is also
broken on multicore machines; on my quad-core system the machine type is
autodetected as 'neon neon neon neon-unknown-linux-gnueabihf':

checking build system type... Invalid configuration `neon': machine `neon' not recognized
configure: error: /nix/store/bafimhdj1yaxj6m1hvq7wvhwwizc939x-bootstrap-tools/bin/sh ./config.sub neon
neon
neon
neon-unknown-linux-gnueabihf failed
builder for ‘/nix/store/1npm2358bpvclj5w7fqjjwg72vbb0d79-gmp-6.0.0a.drv’ failed with exit code 1

Override the system type with the output of GNU config.guess to avoid
the autodetection.

pkgs/development/libraries/gmp/5.1.x.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl, m4, cxx ? true, withStatic ? true }:
 
-with { inherit (stdenv.lib) optional; };
+with { inherit (stdenv.lib) optional optionalString; };
 
 stdenv.mkDerivation rec {
   name = "gmp-5.1.3";
@@ -28,6 +28,13 @@ stdenv.mkDerivation rec {
     ++ optional stdenv.is64bit "--with-pic"
     ;
 
+  # The config.guess in GMP tries to runtime-detect various
+  # ARM optimization flags via /proc/cpuinfo (and is also
+  # broken on multicore CPUs). Avoid this impurity.
+  preConfigure = optionalString stdenv.isArm ''
+      configureFlagsArray+=("--build=$(./configfsf.guess)")
+    '';
+
   doCheck = true;
 
   dontDisableStatic = withStatic;

pkgs/development/libraries/gmp/6.x.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl, m4, cxx ? true, withStatic ? true }:
 
-with { inherit (stdenv.lib) optional; };
+with { inherit (stdenv.lib) optional optionalString; };
 
 stdenv.mkDerivation rec {
   name = "gmp-6.0.0a";
@@ -26,6 +26,13 @@ stdenv.mkDerivation rec {
     ++ optional stdenv.is64bit "--with-pic"
     ;
 
+  # The config.guess in GMP tries to runtime-detect various
+  # ARM optimization flags via /proc/cpuinfo (and is also
+  # broken on multicore CPUs). Avoid this impurity.
+  preConfigure = optionalString stdenv.isArm ''
+      configureFlagsArray+=("--build=$(./configfsf.guess)")
+    '';
+
   doCheck = true;
 
   dontDisableStatic = withStatic;