Explicitly specify /std:c++14 for MSVC

The /std:c++14 option is the default. Explicitly specify it for clarity.

Change-Id: I70218dbeb9f3c1a8b0c2abdf451de537f2ce14ef

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 0118c5d..b4f27a8 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -302,6 +302,9 @@
 
 # Test compiler flags.
 if(MSVC)
+  # It isn't possible to specify C99 conformance for MSVC. MSVC doesn't support
+  # C++ standards modes earlier than C++14.
+  add_cxx_flag_if_supported("/std:c++14")
   add_compiler_flag_if_supported("/W3")
 
   # Disable MSVC warnings that suggest making code non-portable.