jansson: Fix integer and real value formatting with VS2015

This is related to CRT changes in VS2015. All printf related functions
are now defined inline, which breaks the CMake's link time checking.

deps/jansson/CMakeLists.txt

@@ -80,6 +80,7 @@ include (CheckFunctionExists)
 include (CheckFunctionKeywords)
 include (CheckIncludeFiles)
 include (CheckTypeSize)
+include (CheckSymbolExists)
 
 
 if (MSVC)
@@ -221,8 +222,8 @@ else (HAVE_INLINE)
 endif (HAVE_INLINE)
 
 # Find our snprintf
-check_function_exists (snprintf HAVE_SNPRINTF)
-check_function_exists (_snprintf HAVE__SNPRINTF)
+check_symbol_exists(snprintf "stdio.h" HAVE_SNPRINTF)
+check_symbol_exists(_snprintf "stdio.h" HAVE__SNPRINTF)
 
 if (HAVE_SNPRINTF)
    set (JSON_SNPRINTF snprintf)