gmp-*: Clean up configureFlags

pkgs/development/libraries/gmp/4.3.2.nix

@@ -27,8 +27,12 @@ let self = stdenv.mkDerivation rec {
     then "ln -sf configfsf.guess config.guess"
     else ''echo "Darwin host is `./config.guess`."'';
 
-  configureFlags = (if cxx then "--enable-cxx" else "--disable-cxx") +
-    stdenv.lib.optionalString stdenv.isDarwin " ac_cv_build=x86_64-apple-darwin13.4.0 ac_cv_host=x86_64-apple-darwin13.4.0";
+  configureFlags = [
+    (stdenv.lib.enableFeature cxx "cxx")
+  ] ++ stdenv.lib.optionals stdenv.isDarwin [
+    "ac_cv_build=x86_64-apple-darwin13.4.0"
+    "ac_cv_host=x86_64-apple-darwin13.4.0"
+  ];
 
   # The test t-lucnum_ui fails (on Linux/x86_64) when built with GCC 4.8.
   # Newer versions of GMP don't have that issue anymore.

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

@@ -20,18 +20,17 @@ let self = stdenv.mkDerivation rec {
 
   patches = if stdenv.isDarwin then [ ./need-size-t.patch ] else null;
 
-  configureFlags =
+  configureFlags = [
+    "--with-pic"
+    (stdenv.lib.enableFeature cxx "cxx")
     # Build a "fat binary", with routines for several sub-architectures
     # (x86), except on Solaris where some tests crash with "Memory fault".
     # See <http://hydra.nixos.org/build/2760931>, for instance.
     #
     # no darwin because gmp uses ASM that clang doesn't like
-    optional (!stdenv.isSunOS) "--enable-fat"
-    ++ (if cxx then [ "--enable-cxx"  ]
-               else [ "--disable-cxx" ])
-    ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
-    ++ optional stdenv.isDarwin "ABI=64"
-    ++ optional stdenv.is64bit "--with-pic"
+    (stdenv.lib.enableFeature (!stdenv.isSunOS && stdenv.hostPlatform.isx86) "fat")
+  ] ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
+    ++ optional (stdenv.isDarwin && stdenv.is64bit) "ABI=64"
     ;
 
   # The config.guess in GMP tries to runtime-detect various

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

@@ -21,18 +21,17 @@ let self = stdenv.mkDerivation rec {
   depsBuildBuild = [ buildPackages.stdenv.cc ];
   nativeBuildInputs = [ m4 ];
 
-  configureFlags =
+  configureFlags = [
+    "--with-pic"
+    (stdenv.lib.enableFeature cxx "cxx")
     # Build a "fat binary", with routines for several sub-architectures
     # (x86), except on Solaris where some tests crash with "Memory fault".
     # See <http://hydra.nixos.org/build/2760931>, for instance.
     #
     # no darwin because gmp uses ASM that clang doesn't like
-    optional (!stdenv.isSunOS && stdenv.hostPlatform.isx86) "--enable-fat"
-    ++ (if cxx then [ "--enable-cxx"  ]
-               else [ "--disable-cxx" ])
-    ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
+    (stdenv.lib.enableFeature (!stdenv.isSunOS && stdenv.hostPlatform.isx86) "fat")
+  ] ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
     ++ optional (stdenv.isDarwin && stdenv.is64bit) "ABI=64"
-    ++ optional stdenv.is64bit "--with-pic"
     ++ optional (with stdenv.hostPlatform; useAndroidPrebuilt || useiOSPrebuilt) "--disable-assembly"
     ;