Fix MSVC warning C4273 "inconsistent dll linkage"

When using CMake, the user can select the MSVC runtime library to be:
1) Statically-linked (with the corresponding `x64-windows-static` vcpkg
triplet) or
2) Dynamically-linked (with the corresponding `x64-windows` vcpkg
triplet)

In the latter case, the compiler emits the C4273 warning.

As the "Necessary on some platforms" comment does not apply to MSVC,
skip the declaration for MSVC.

src/randomenv.cpp

@@ -58,7 +58,9 @@
 #include <sys/auxv.h>
 #endif
 
+#ifndef _MSC_VER
 extern char** environ; // NOLINT(readability-redundant-declaration): Necessary on some platforms
+#endif
 
 namespace {