Windows: Always define _UCRT when compiling C code

As seen in #22159, this is required to ensure correct behavior when MinGW-w64
headers are in the `C_INCLUDE_PATH`.

Fixes #22159.

m4/fp_setup_windows_toolchain.m4

@@ -82,7 +82,11 @@ AC_DEFUN([FP_SETUP_WINDOWS_TOOLCHAIN],[
 
     CC="${mingwbin}clang.exe"
     CXX="${mingwbin}clang++.exe"
-    cflags="--rtlib=compiler-rt"
+
+    # Signal that we are linking against UCRT with the _UCRT macro. This is
+    # necessary to ensure correct behavior when MinGW-w64 headers are in the
+    # header include path (#22159).
+    cflags="--rtlib=compiler-rt -D_UCRT"
     CFLAGS="$cflags"
     CONF_CC_OPTS_STAGE1="$cflags"
     CONF_CC_OPTS_STAGE2="$cflags"

testsuite/tests/ffi/should_run/Makefile

@@ -49,3 +49,10 @@ T15933:
 	'$(TEST_HC)' $(TEST_HC_OPTS) -c T15933.hs
 	'$(TEST_HC)' $(TEST_HC_OPTS) T15933_c.o T15933.o -o T15933
 	./T15933
+
+.PHONY: T22159
+T22159:
+	C_INCLUDE_PATH=/mingw64/include '$(TEST_HC)' $(TEST_HC_OPTS) -c T22159.hs
+	C_INCLUDE_PATH=/mingw64/include '$(TEST_HC)' $(TEST_HC_OPTS) -c T22159_c.c
+	C_INCLUDE_PATH=/mingw64/include '$(TEST_HC)' $(TEST_HC_OPTS) T22159.o T22159_c.o -o T22159
+	./T22159

testsuite/tests/ffi/should_run/T22159.hs

+{-# LANGUAGE CPP #-}
+module Main (main) where
+
+#if defined(i386_HOST_ARCH)
+# define WINDOWS_CCONV stdcall
+#elif defined(x86_64_HOST_ARCH)
+# define WINDOWS_CCONV ccall
+#else
+# error Unknown mingw32 arch
+#endif
+
+import Foreign.C.String (peekCWString)
+import Foreign.C.Types (CWchar)
+import Foreign.Marshal.Alloc (allocaBytes)
+import Foreign.Ptr (Ptr)
+
+foreign import WINDOWS_CCONV "hello" c_hello :: Ptr CWchar -> IO ()
+
+main :: IO ()
+main = allocaBytes 12 $ \buf -> do
+  c_hello buf
+  str <- peekCWString buf
+  putStrLn str

testsuite/tests/ffi/should_run/T22159.stdout

+hello

testsuite/tests/ffi/should_run/T22159_c.c

+#include <stdio.h>
+#include <wchar.h>
+
+void hello(wchar_t *buf) {
+  swprintf_s(buf, 12, L"hello");
+}

testsuite/tests/ffi/should_run/all.T

@@ -229,3 +229,8 @@ test('T19237', normal, compile_and_run, ['T19237_c.c'])
 
 test('T21305', omit_ways(['ghci']), multi_compile_and_run,
                  ['T21305', [('T21305_cmm.cmm', '')], ''])
+
+test('T22159',
+     [unless(opsys('mingw32'), skip),
+      extra_files(['T22159_c.c'])],
+     makefile_test, ['T22159'])