Specify c++ standard to use for building libktx. (KhronosGroup#806)

Avoid inadvertent compilation with a later standard
when included in another project. Later standards may
raise warnings or even errors on the library code. 

Fixes KhronosGroup#774 and KhronosGroup#656.

CMakeLists.txt

@@ -470,6 +470,9 @@ macro(common_libktx_settings target enable_write library_type)
         VERSION ${PROJECT_VERSION}
         SOVERSION ${PROJECT_VERSION_MAJOR}
         XCODE_ATTRIBUTE_ENABLE_HARDENED_RUNTIME "YES"
+        CXX_STANDARD 11
+        CXX_STANDARD_REQUIRED YES
+
     )
     if(IOS)
         set_target_properties(${target} PROPERTIES