these are poorly designed (illogical argument order) and even poorly
implemented (brace issues) on glibc, but unfortunately some software
is using them. we could consider removing them again in the future at
some point if they're documented as deprecated, but for now the
simplest thing to do is just to provide them under _GNU_SOURCE.

previously:
timersub(&now, t, &diff);

warning: value computed is not used [-Wunused-value]

this function is obsolete, however it's available as a syscall
and as such qemu userspace emulation tries to forward it to the
host kernel.

the old behavior of exposing nothing except plain ISO C can be
obtained by defining __STRICT_ANSI__ or using a compiler option (such
as -std=c99) that predefines it. the new default featureset is POSIX
with XSI plus _BSD_SOURCE. any explicit feature test macros will
inhibit the default.

installation docs have also been updated to reflect this change.

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

patch by Isaac Dunham. matched closely (maybe not exact) to glibc's
idea of what _BSD_SOURCE should make visible.

based on patch by sh4rm4. these functions are deprecated; futimens and
utimensat should be used instead in new programs.

trash in the upper 32 bits was making the kernel sleep forever in
select on 64-bit systems.

thanks to Peter Mazinger (psm) for pointing many of these issues out
and submitting a patch on which this commit is loosely based