remove unused BITOPS_DEFINE
build: Enable C99, and no longer forbid declaration-after-statement

gcc has supported the C99 feature declaration-after-statement
since 3.0(!): http://gcc.gnu.org/gcc-3.0/c99status.html

as a result, any version of gcc plausibly being used today will support
this language feature at least when the relevant flag (e.g., -std=c99)
is passed and/or the relevant warning is silenced (e.g., -Wno-declaration-
after-statement)

In some versions of rtai, rtai_lxrt.h includes extern inlines which refer
to pthread functions.  We require that these function bodies not be
emitted into the object file when they are unused; this is accomplished
by the -fgnu89-inline flag, which uses the old inline behavior even when
c99/gnu99 mode is enabled.