Create randn_sfmt_posix_7.c

This is a two thread, posix Mersenne, ziggurat, Gaussian random generator verson of the single thread randn_sfmt.c.  I uses one thread for the real part an one for the complex part of the 2-D matrix double complex array[n][n], but is created in 1-D [n*n] for speed, and then uses  a C union of the 1-D *ptr array and  2-D  **prt  for the 2-D, complex  matrix.

This is the fastest posix pthread_create() version to date. It uses c global variables (Fortran Common block arrays) to avoid the awkwardness and slow passing of pointers byposix  explicitly in pthread_create():

pthread_create(&tid_0, NULL, zig_core_0, (void *) NULL);
pthread_create(&tid_1, NULL, zig_core_1, (void *) NULL);

rather than the slower, but more typical:

pthread_create(&tid_0, NULL, zig_core_0, (void *) &thdata);		
pthread_create(&tid_1, NULL, zig_core_1, (void *) &thdata);

At this point the overhead of posix makes this routine 20% slower than the single thread randn_sfmt.c.   Suggestions for improvement are solicited.