CMake: Try to set a required try_run() definition in a more idiomatic way

This is applicable to every test (if we had more), not just HAVE_C99_VSNPRINTF.

Could also be a #define but let's go with this for now. This takes
advantage of the stack based design of CMakePushCheckState.

CMakeLists.txt

@@ -472,6 +472,7 @@ if(MINGW)
 		#
 		-D_POSIX
 	)
+	list(APPEND CMAKE_REQUIRED_DEFINITIONS -D_POSIX)
 endif()
 
 if( CMAKE_C_COMPILER_ID MATCHES "MSVC")

ConfigureChecks.cmake

@@ -107,10 +107,6 @@ check_symbol_exists(tzname "time.h" HAVE_TZNAME)
 # Check if the libc vsnprintf() conforms to C99. If this fails we may
 # need to fall-back on GLib I/O.
 #
-cmake_push_check_state(RESET)
-if(MINGW)
-	set(CMAKE_REQUIRED_DEFINITIONS -D__USE_MINGW_ANSI_STDIO)
-endif()
 check_c_source_runs("
 	#include <stdio.h>
 	int main(void)
@@ -124,7 +120,6 @@ check_c_source_runs("
 	}"
 	HAVE_C99_VSNPRINTF
 )
-cmake_pop_check_state()
 if (NOT HAVE_C99_VSNPRINTF)
 	message(FATAL_ERROR
 "Building Wireshark requires a C99 compliant vsnprintf() and this \