Compile error with gcc

[Garrett Cooper]

Hi Julio,
	I ran into this error when trying to compile ATF with gcc on code based off of r261958:

--- application.So ---
/usr/src/contrib/atf/atf-c++/detail/application.cpp: In constructor 'atf::application::usage_error::usage_error(const char*, ...)':
/usr/src/contrib/atf/atf-c++/detail/application.cpp:71: error: 'vsnprintf' is not a member of 'std'
*** [application.So] Error code 1

	I haven’t dug into why this is occurring yet, but I assume it’s because of the ATF 0.20 upgrade.
	I’m compiling with gcc/g++ of course, and not clang/libcxxrt.
Thanks!
-Garrett