It is a subtle defect with function:
int vsnprintf (char *str, size_t count, const char *fmt, va_list args)

which is defined in module lib/snprintf.c
If compiler is not compliant with C99, then this function is defined and
overrides standard implementation.
Defect happens when you try to link Samba library with any program which loads
standard libraries and uses standard vsnprintf. Solaris loads function only 
once, so Samba appears to use _standard_ function, not which is defined in 
Samba.
Defect happens only if Samba library is loaded after program uses this 
function. 
Simple workaround to this is to change the name of this function to something 
like:
int smb_vsnprintf (char *str, size_t count, const char *fmt, va_list args)

and then use
#define vsnprintf smb_vsnprintf

By the way, other "printf" functions are substituted in this (correct) way.