Mira build failures in darshan_core_printf() function due to va_end and va_start

From nightly tests:

/gpfs/mira-home/radixio/cinow-01/workspace/Radix-IO/Darshan/x86-master-profconf/install/lib/libdarshan.a(darshan-core.o): In function `darshan_core_fprintf':
/gpfs/mira-home/radixio/cinow-01/workspace/Radix-IO/Darshan/x86-master-profconf/build/darshan-runtime/../../darshan-runtime/lib/darshan-core.c:2148: undefined reference to `va_start'
/gpfs/mira-home/radixio/cinow-01/workspace/Radix-IO/Darshan/x86-master-profconf/build/darshan-runtime/../../darshan-runtime/lib/darshan-core.c:2150: undefined reference to `va_end'
collect2: error: ld returned 1 exit status

This doesn't happen on Ubuntu; need to determine what the difference is here.