Bump iOS SDK version to the current one, 11.4

Not sure why we at some stage lost the possibiliy to build against any
of several recent versions, but require one specific. But yeah, no big
deal, anybody working on iOS code is expected to keep ther Xcode (and
thus SDKs) updated.

README.md

@@ -42,8 +42,8 @@ run and compile LibreOffice, also used by the TDF builds:
     * Runtime: RHEL 6 or CentOS 6
     * Build: GCC 4.8.1 or Clang
 * iOS (only for LibreOfficeKit):
-    * Runtime: 11.3 (only support for newer i devices == 64 bit)
-    * Build: Xcode 9.3 and iPhone SDK 11.3
+    * Runtime: 11.4 (only support for newer i devices == 64 bit)
+    * Build: Xcode 9.3 and iPhone SDK 11.4
 
 At least Clang 3.4.2 is known to be too old to pass the configure.ac check "whether $CXX supports
 C++17, C++14, or C++11" in its current form (due to the #pragma GCC diagnostic ignored "-Wpragmas"

configure.ac

@@ -2932,7 +2932,7 @@ dnl ===================================================================
 
 if test $_os = iOS; then
     AC_MSG_CHECKING([what iOS SDK to use])
-    current_sdk_ver=11.3
+    current_sdk_ver=11.4
     if test "$enable_ios_simulator" = "yes"; then
         platform=iPhoneSimulator
         versionmin=-mios-simulator-version-min=$current_sdk_ver