Merge bitcoin#30491: Fix MSVC warning C4273 "inconsistent dll linkage"

7703884 Fix MSVC warning C4273 "inconsistent dll linkage" (Hennadii Stepanov)

Pull request description:

  Broken out of bitcoin#30454.

  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](https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4273) warning.

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

  The MSVC build system in the master branch supports the statically-linked runtime only: https://github.com/bitcoin/bitcoin/blob/ed739d14b58b5e772a65b85bb421703963b06852/build_msvc/common.init.vcxproj.in#L65

ACKs for top commit:
  sipa:
    utACK 7703884
  sipsorcery:
    utACK 7703884.
  theuni:
    utACK 7703884

Tree-SHA512: a42e1a0d48973217462e703c418f3e9ef9cb5236267c1bf32912aacaf68976cdd2b9229168523f7c2a99ee3f2fb1bf8add4f342796bdb1e4063ca026b761db51

Author: fanquake

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 {