Fix clang warnings

This should fix most of the clang warnings for default plugins.

One warning I wasn't able to fix is:
./src/include/kdberrors.h:41:42: warning: token pasting of ',' and \_\_VA_ARGS\_\_ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]

There's a discussion about a standard alternative to \_\_VA_ARGS\_\_ at http://stackoverflow.com/questions/5588855/standard-alternative-to-gccs-va-args-trick but the argument counting seems awkward.

I'll look into the nodep-plugins now.
Some, even relatively new, compilers do not support _Pragma.
Could you please give it in a #ifdef like it is done with ELEKTRA_GCC_WARNING

Maybe you can even use ELEKTRA_GCC_WARNING? In that case we just need a more general name, e.g. ELEKTRA_PRAGMA