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: <sys/socket.h> not defining MSG_NOSIGNAL.
  - DPOSIXFAIL_CLOCK_REALTIME: <time.h> not defining CLOCK_REALTIME.
  - DPOSIXFAIL_CLOCK_GETTIME: <time.h> 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
