test for vsnprintf in BuildSystem

[Lisandro Dalcin]

Satish, do you remember why?:

$ hg log -r 2219
changeset:   2219:b2d7f7d19b64
parent:      2217:08f36f5dec9b
user:        balay at mcs.anl.gov
date:        Fri Jan 30 17:53:26 2009 -0600
summary:     WIndows fix. If '_vsnprintf' exists - then don't check
for HAVE_VSNPRINTF_CHAR

I'm getting this warning when building on Cygwin (with GCC):

libfast in: /cygdrive/c/DEVEL/petsc-dev-cygwin/src/sys/fileio
mprint.c: In function 'PetscVSNPrintf':
mprint.c:106: warning: implicit declaration of function '_vsnprintf'


-- 
Lisandro Dalcín
---------------
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
PTLC - Güemes 3450, (3000) Santa Fe, Argentina
Tel/Fax: +54-(0)342-451.1594