Make sure _MSC_VER is defined before checking (elalish#990)

Avoids the following:

error: "_MSC_VER" is not defined, evaluates to 0 [-Werror=undef]
   59 | #if _MSC_VER <= 1900
      |     ^~~~~~~~

include/manifold/linalg.h

@@ -56,7 +56,7 @@
 
 // In Visual Studio 2015, `constexpr` applied to a member function implies
 // `const`, which causes ambiguous overload resolution
-#if _MSC_VER <= 1900
+#if defined(_MSC_VER) && (_MSC_VER <= 1900)
 #define LINALG_CONSTEXPR14
 #else
 #define LINALG_CONSTEXPR14 constexpr