[1991] exclude clang++ for the check explicitly to avoid false positives

this can happen with clang++ used via ccache

configure.ac

@@ -846,8 +846,9 @@ if test "$BOOST_OFFSET_PTR_FAILURE" = "yes"; then
 fi
 
 # There's a known bug in FreeBSD ports for Boost that would trigger a false
-# warning in build with g++ and -Werror
-if test "$BOOST_NUMERIC_CAST_WOULDFAIL" = "yes" -a X"$with_werror" = X1; then
+# warning in build with g++ and -Werror (we exclude clang++ explicitly to
+# avoid unexpected false positives).
+if test "$BOOST_NUMERIC_CAST_WOULDFAIL" = "yes" -a X"$with_werror" = X1 -a $CLANGPP = "no"; then
     AC_MSG_ERROR([Failed to compile a required header file.  If you are using FreeBSD and Boost installed via ports, retry with specifying --without-werror.  See the ChangeLog entry for Trac no. 1991 for more details.])
 fi