warn in configure about using ccache+clang without CCACHE_CPP2 set

- causes warnings about unused -I options, because they are preprocessing options
  and Clang complains that they are unused in pure compilation
- Clang supresses some warnings from expansions of macros in system headers, but those
  would be already expanded and thus warned about
- Clang prints relevant source lines with errors/warnings in them and would show
  them preprocessed (this should actually apply to GCC4.8+ too)

Change-Id: I02c86c43a657d078e347e4ecd5c1e2ff07db2886

configure.ac

@@ -2896,6 +2896,13 @@ if test "$GCC" = "yes"; then
 fi
 AC_SUBST(COM_GCC_IS_CLANG)
 
+if test "$CCACHE" != "" -a "$COM_GCC_IS_CLANG" = TRUE; then
+    if test -z "$CCACHE_CPP2"; then
+            AC_MSG_WARN([Using ccache with Clang without CCACHE_CPP2 set causes spurious warnings.])
+            add_warning "Using ccache with Clang without CCACHE_CPP2 set causes spurious warnings."
+    fi
+fi
+
 # ===================================================================
 # check various GCC options that Clang does not support now but maybe
 # will somewhen in the future, check them even for GCC, so that the