iOS SDK 12.0 is the current now

Also, I don't see why we would want to use -miphoneos-version-min=12.0
just because we build against the 12.0 SDK. Use 11.0 as the minimum
runtime iOS version for now.

Change-Id: I36b9696ce367249e7e8d0a51ee61791e32972390

configure.ac

@@ -2946,13 +2946,13 @@ dnl ===================================================================
 
 if test $_os = iOS; then
     AC_MSG_CHECKING([what iOS SDK to use])
-    current_sdk_ver=11.4
+    current_sdk_ver=12.0
     if test "$enable_ios_simulator" = "yes"; then
         platform=iPhoneSimulator
         versionmin=-mios-simulator-version-min=$current_sdk_ver
     else
         platform=iPhoneOS
-        versionmin=-miphoneos-version-min=$current_sdk_ver
+        versionmin=-miphoneos-version-min=11.0
     fi
     xcode_developer=`xcode-select -print-path`