Silence unhelpful -Wpotentially-evaluated-expression

...on Clang trunk towards 3.6, firing for typeid(*e) where e is a side-effecting
expression (of polymorphic pointer type).  Simpler to disable it via #if in
sal/config.h than to disable it in solenv/gbuild/platform/com_GCC_defs.mk with
an additional feature test in configure.ac.

Change-Id: If94692a9e06ff2659bf168b4968200aeee9ebb0a

include/sal/config.h

@@ -86,6 +86,12 @@
 #define SAL_CONFIGFILE( name ) name "rc"
 #endif
 
+#if defined __clang__
+#if __has_warning("-Wpotentially-evaluated-expression")
+#pragma GCC diagnostic ignored "-Wpotentially-evaluated-expression"
+#endif
+#endif
+
 #endif // INCLUDED_SAL_CONFIG_H