scrypt/libcperciva/POSIX

POSIX compatibility code
------------------------

This code exists to work around some common POSIX compatibility issues.
POSIX specifies that if the first line of a Makefile is ".POSIX:" then the
Makefile should be processed according to POSIX rules, including with CC=c99;
thus we should see behaviour consistent with a standard-compliant C99
compiler.


- posix-cflags.sh: Detects if ${CC} supports certain POSIX features, and
outputs a POSIXFAIL_ define if it is not supported so that we can work around
the problem.

The potential command-line flags are:
- DPOSIXFAIL_MSG_NOSIGNAL: not defining MSG_NOSIGNAL.
- DPOSIXFAIL_CLOCK_REALTIME: not defining CLOCK_REALTIME.
- DPOSIXFAIL_CLOCK_GETTIME: not declaring clock_gettime(), or
clock_gettime() is not linkable.
- std=c99: ${CC} does not accept the `restrict` keyword by default, but
accepts it when given this flag.

posix-l.sh: Detects whether the linker supports certain POSIX features.

The potential command-line flags are:
- lrt lxnet: c99 is required to understand these options, and ignore them if
the routines for which they specify linkage are already in the standard C
library