Various -Wextra fixups

It appears that some more errors have either creeped into the Chisel
C++ header files or into GCC's "-Wextra -pedantic" set.  This patch
uses the same techniques as last time to hide these new errors.
clang and gcc do not quite agree on -Wpragmas.
Merge pull request #345 from ucb-bar/pr/341

Clang/gcc inconsistencies with -Wpragmas.