Avoid ccache/clang warnings

build/autotools/configure.ac

@@ -46,7 +46,7 @@ AC_CHECK_PROG(OBJCOPY, objcopy, [objcopy --remove-section=.note --remove-section
 AC_CHECK_PROG(STRIP, strip, strip, touch)
 AC_CHECK_PROG(DIFF, gdiff, gdiff, diff)
 AC_CHECK_PROG(UNAME, uname, uname)
-AC_CHECK_PROG(CCACHE, ccache, ccache)
+AC_CHECK_PROG(CCACHE, ccache, env CCACHE_CPP2=1 ccache)
 AC_CHECK_PROG(DISTCC, distcc, distcc)
 
 # Test the os and set the module linking settings

configure

@@ -3919,7 +3919,7 @@ do
   test -z "$as_dir" && as_dir=.
     for ac_exec_ext in '' $ac_executable_extensions; do
   if as_fn_executable_p "$as_dir/$ac_word$ac_exec_ext"; then
-    ac_cv_prog_CCACHE="ccache"
+    ac_cv_prog_CCACHE="env CCACHE_CPP2=1 ccache"
     $as_echo "$as_me:${as_lineno-$LINENO}: found $as_dir/$ac_word$ac_exec_ext" >&5
     break 2
   fi