build: use c99 code build option
    
    fix build on centos7/rhel7:
    
    06:51:31 rgrp.c: In function 'lgfs2_rgrp_read':
    06:51:31 rgrp.c:208:2: error: 'for' loop initial declarations are only
       allowed in C99 mode
    06:51:31   for (unsigned i = 0; i < rgd->rt_length; i++) {
    06:51:31   ^
    06:51:31 rgrp.c:208:2: note: use option -std=c99 or -std=gnu99 to
       compile your code
    06:51:31 rgrp.c: In function 'lgfs2_rgrp_relse':
    06:51:31 rgrp.c:234:2: error: 'for' loop initial declarations are only
       allowed in C99 mode
    06:51:31   for (unsigned i = 0; i < rgd->rt_length; i++) {
    06:51:31   ^
    06:51:31 rgrp.c:249:16: error: redefinition of 'i'
    06:51:31   for (unsigned i = 0; i < rgd->rt_length; i++)
    06:51:31                 ^
    06:51:31 rgrp.c:234:16: note: previous definition of 'i' was here
    06:51:31   for (unsigned i = 0; i < rgd->rt_length; i++) {
    06:51:31                 ^
    06:51:31 rgrp.c:249:2: error: 'for' loop initial declarations are only
       allowed in C99 mode
    06:51:31   for (unsigned i = 0; i < rgd->rt_length; i++)
    06:51:31   ^
    
    and plenty other spread across the code
    
    Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>