MSVC: Prepare for MinGW-w64-based libs support

I got a std.stdio hello-world executable to work on Win64 using the
Visual C++ 2015 DLLs, with both MS linker and -link-internally, see
dlang/installer#346.

It still requires a couple of extra command-line args, but will in the
end enable us to get rid of all external dependencies for building on
Windows (via -link-internally and the MinGW-w64-based import libs, which
will only add a few MB to the prebuilt packages).

The Visual C++ 2015 *runtime* will need to be installed when running
binaries linked against the MinGW libs.

Author: kinke

driver/linker-msvc.cpp

@@ -36,6 +36,13 @@ void addMscrtLibs(std::vector<std::string> &args) {
 
   args.push_back(("/DEFAULTLIB:" + mscrtlibName).str());
 
+  // We need the vcruntime lib for druntime's exception handling (ldc.eh_msvc).
+  // Pick one of the 4 variants matching the selected main UCRT lib.
+
+  if (mscrtlibName.contains_lower("vcruntime")) {
+    return;
+  }
+
   const bool isStatic = mscrtlibName.startswith_lower("libcmt");
   const bool isDebug =
       mscrtlibName.endswith_lower("d") || mscrtlibName.endswith_lower("d.lib");

runtime/CMakeLists.txt

@@ -307,6 +307,9 @@ if("${TARGET_SYSTEM}" MATCHES "MSVC")
     # We want to be able to link against all 4 C runtime variants (libcmt[d] / msvcrt[d]).
     string(REGEX REPLACE "(^| )[/-]M[TD]d?( |$)" "\\2" CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}")
     append("/MT /Zl" CMAKE_C_FLAGS_RELEASE)
+    # Disable buffer overflow checks to prevent dependencies on special C symbols
+    # not available in the MinGW-w64 libs.
+    append("/GS-" CMAKE_C_FLAGS_RELEASE)
     # warning C4100: unreferenced formal parameter
     # warning C4127: conditional expression is constant
     # warning C4131: uses old-style declarator