diff --git a/configure b/configure
index 56c78fc6295..d1edbdc684a 100755
--- a/configure
+++ b/configure
@@ -17613,7 +17613,8 @@ if test "x$ac_cv_cflags__Wl__delayload_autoconftest_dll" = xyes; then :
   DELAYLOADFLAG="-Wl,-delayload,"
 
 fi ;;
-    *) { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports -fno-builtin" >&5
+    *) MSVCRTFLAGS="-D_WIN32"
+       { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the compiler supports -fno-builtin" >&5
 $as_echo_n "checking whether the compiler supports -fno-builtin... " >&6; }
 if ${ac_cv_cflags__fno_builtin+:} false; then :
   $as_echo_n "(cached) " >&6
diff --git a/configure.ac b/configure.ac
index af1cf7303ba..da6ecf5a7a8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2045,7 +2045,8 @@ char*f(const char *h,char n) {return strchr(h,n);}]])],[ac_cv_c_logicalop_noisy=
   case $host_os in
     mingw32*)  dnl Check for delayload linker support
        WINE_TRY_CFLAGS([-Wl,-delayload,autoconftest.dll],[AC_SUBST(DELAYLOADFLAG,["-Wl,-delayload,"])]) ;;
-    *) WINE_TRY_CFLAGS([-fno-builtin],[MSVCRTFLAGS="$MSVCRTFLAGS -fno-builtin"])
+    *) MSVCRTFLAGS="-D_WIN32"
+       WINE_TRY_CFLAGS([-fno-builtin],[MSVCRTFLAGS="$MSVCRTFLAGS -fno-builtin"])
        WINE_TRY_CFLAGS([-fshort-wchar],[MSVCRTFLAGS="$MSVCRTFLAGS -fshort-wchar"]) ;;
   esac
 
diff --git a/include/msvcrt/corecrt.h b/include/msvcrt/corecrt.h
index a02feb509e8..8f407bdb20c 100644
--- a/include/msvcrt/corecrt.h
+++ b/include/msvcrt/corecrt.h
@@ -25,8 +25,16 @@
 #define __WINE_USE_MSVCRT
 #endif
 
-#ifdef __WINE_WINE_PORT_H
-# error You cannot use both wine/port.h and msvcrt headers
+#ifdef __WINE_CONFIG_H
+# error You cannot use config.h with msvcrt
+#endif
+
+#ifndef _WIN32
+# define _WIN32
+#endif
+
+#ifndef WIN32
+# define WIN32
 #endif
 
 #if (defined(__x86_64__) || defined(__powerpc64__) || defined(__aarch64__)) && !defined(_WIN64)