-mcrtdll not necessary

.github/workflows/mingw-w64.yml

@@ -80,7 +80,7 @@ jobs:
         shell: msys2 {0}
         run: |
           mkdir bin
-          cc crystal.obj -o bin/crystal.exe -mcrtdll=ucrt -municode \
+          cc crystal.obj -o bin/crystal.exe -municode \
             $(pkg-config bdw-gc libpcre2-8 iconv zlib libffi --libs) \
             $(llvm-config --libs --system-libs --ldflags) \
             -lDbgHelp -lole32 -lWS2_32 -Wl,--stack,0x800000

src/compiler/crystal/loader/mingw.cr

@@ -7,6 +7,8 @@ require "crystal/system/win32/library_archive"
 # The core implementation is derived from the MSVC loader. Main deviations are:
 #
 # - `.parse` follows GNU `ld`'s style, rather than MSVC `link`'s;
+# - `.parse` automatically inserts a C runtime library if `-mcrtdll` isn't
+#   supplied;
 # - `#library_filename` follows the usual naming of the MinGW linker: `.dll.a`
 #   for DLL import libraries, `.a` for other libraries;
 # - `.default_search_paths` relies solely on `.cc_each_library_path`.
@@ -27,6 +29,10 @@ class Crystal::Loader
     libnames = [] of String
     file_paths = [] of String
     extra_search_paths = [] of String
+
+    # note that `msvcrt` is a default runtime chosen at MinGW-w64 build time,
+    # `ucrt` is always UCRT (even in a MINGW64 environment), and
+    # `msvcrt-os` is always MSVCRT (even in a UCRT64 environment)
     crt_dll = "msvcrt"
 
     OptionParser.parse(args.dup) do |parser|

src/lib_c.cr

@@ -1,7 +1,5 @@
 {% if flag?(:msvc) %}
   @[Link({{ flag?(:static) ? "libucrt" : "ucrt" }})]
-{% elsif flag?(:win32) && flag?(:gnu) %}
-  @[Link(ldflags: "-mcrtdll=ucrt")]
 {% end %}
 lib LibC
   alias Char = UInt8