gmp: don't infer build platform from uname

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

@@ -29,17 +29,14 @@ let self = stdenv.mkDerivation rec {
     #
     # no darwin because gmp uses ASM that clang doesn't like
     (stdenv.lib.enableFeature (!stdenv.isSunOS && stdenv.hostPlatform.isx86) "fat")
+    # 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.
+    "--build=${stdenv.buildPlatform.config}"
   ] ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
     ++ optional (stdenv.isDarwin && stdenv.is64bit) "ABI=64"
     ;
 
-  # 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.isAarch32 ''
-      configureFlagsArray+=("--build=$(./configfsf.guess)")
-    '';
-
   doCheck = true;
 
   dontDisableStatic = withStatic;

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

@@ -30,18 +30,15 @@ let self = stdenv.mkDerivation rec {
     #
     # no darwin because gmp uses ASM that clang doesn't like
     (stdenv.lib.enableFeature (!stdenv.isSunOS && stdenv.hostPlatform.isx86) "fat")
+    # 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.
+    "--build=${stdenv.buildPlatform.config}"
   ] ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
     ++ optional (stdenv.isDarwin && stdenv.is64bit) "ABI=64"
     ++ optional (with stdenv.hostPlatform; useAndroidPrebuilt || useiOSPrebuilt) "--disable-assembly"
     ;
 
-  # 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.isAarch32 ''
-      configureFlagsArray+=("--build=$(./configfsf.guess)")
-    '';
-
   doCheck = true; # not cross;
 
   dontDisableStatic = withStatic;