xcodebuild: fix to make it build against the 11.2 iphone SDK

pkgs/development/mobile/xcodeenv/build-app.nix

@@ -1,7 +1,7 @@
 {stdenv, xcodewrapper}:
 { name
 , src
-, sdkVersion ? "10.2"
+, sdkVersion ? "11.2"
 , target ? null
 , configuration ? null
 , scheme ? null
@@ -80,8 +80,10 @@ stdenv.mkDerivation {
       ''}
 
     # Do the building
+    export LD=clang # To avoid problem with -isysroot parameter that is unrecognized by the stock ld. Comparison with an impure build shows that it uses clang instead. Ugly, but it works
+
     xcodebuild -target ${_target} -configuration ${_configuration} ${stdenv.lib.optionalString (scheme != null) "-scheme ${scheme}"} -sdk ${_sdk} TARGETED_DEVICE_FAMILY="1, 2" ONLY_ACTIVE_ARCH=NO CONFIGURATION_TEMP_DIR=$TMPDIR CONFIGURATION_BUILD_DIR=$out ${if generateXCArchive then "archive" else ""} ${xcodeFlags} ${if release then ''"CODE_SIGN_IDENTITY=${codeSignIdentity}" PROVISIONING_PROFILE=$PROVISIONING_PROFILE OTHER_CODE_SIGN_FLAGS="--keychain $HOME/Library/Keychains/$keychainName-db"'' else ""}
-    
+
     ${stdenv.lib.optionalString release ''
       ${stdenv.lib.optionalString generateIPA ''
         # Produce an IPA file