gcc: use Nix instead of bash for conditional

Now that we use the standard builder, the commands produced by
pre-configure.nix are wrapped in a bash function.  Inside of a bash
function, `export foo=` will still add `foo` to the environment of
any child processes forked after that point, but those variables
will *not* be visible to bash code which is outside of the
function-scope in which the `export` occurs.

Weird crap like this is yet another reason why we need to move away
from using bash for logic.  Let's switch.

pkgs/development/compilers/gcc/common/builder.nix

@@ -1,5 +1,6 @@
 { lib
 , stdenv
+, enableMultilib
 }:
 
 originalAttrs: (stdenv.mkDerivation (finalAttrs: originalAttrs // {
@@ -188,16 +189,22 @@ originalAttrs: (stdenv.mkDerivation (finalAttrs: originalAttrs // {
   preInstall = ''
     mkdir -p "$out/''${targetConfig}/lib"
     mkdir -p "''${!outputLib}/''${targetConfig}/lib"
-    # Make ‘lib64’ symlinks to ‘lib’.
-    if [ -n "$linkLib64toLib" ]; then
-        ln -s lib "$out/''${targetConfig}/lib64"
-        ln -s lib "''${!outputLib}/''${targetConfig}/lib64"
-    fi
-    # Make ‘lib32’ symlinks to ‘lib’.
-    if [ -n "$linkLib32toLib" ]; then
-        ln -s lib "$out/''${targetConfig}/lib32"
-        ln -s lib "''${!outputLib}/''${targetConfig}/lib32"
-    fi
+  '' +
+  # Make `lib64` symlinks to `lib`.
+  lib.optionalString (!enableMultilib && stdenv.hostPlatform.is64bit && !stdenv.hostPlatform.isMips64n32) ''
+    ln -s lib "$out/''${targetConfig}/lib64"
+    ln -s lib "''${!outputLib}/''${targetConfig}/lib64"
+  '' +
+  # On mips platforms, gcc follows the IRIX naming convention:
+  #
+  #  $PREFIX/lib   = mips32
+  #  $PREFIX/lib32 = mips64n32
+  #  $PREFIX/lib64 = mips64
+  #
+  # Make `lib32` symlinks to `lib`.
+  lib.optionalString (!enableMultilib && stdenv.targetPlatform.isMips64n32) ''
+    ln -s lib "$out/''${targetConfig}/lib32"
+    ln -s lib "''${!outputLib}/''${targetConfig}/lib32"
   '';
 
   postInstall = ''

pkgs/development/compilers/gcc/common/pre-configure.nix

@@ -148,16 +148,3 @@ in lib.optionalString (hostPlatform.isSunOS && hostPlatform.is64bit) ''
     echo 'SHLIB_LC=${SHLIB_LC}' >> libgcc/Makefile.in
   '')
 
-+ lib.optionalString (!enableMultilib && hostPlatform.is64bit && !hostPlatform.isMips64n32) ''
-  export linkLib64toLib=1
-''
-
-# On mips platforms, gcc follows the IRIX naming convention:
-#
-#  $PREFIX/lib   = mips32
-#  $PREFIX/lib32 = mips64n32
-#  $PREFIX/lib64 = mips64
-#
-+ lib.optionalString (!enableMultilib && targetPlatform.isMips64n32) ''
-  export linkLib32toLib=1
-''