string, wchar: Fix compilation error on MSVC (again)

[Bruno Haible]

Compiling Reuben's libpaper-1.2.0 tarball on MSVC, I get this compilation
error:

  CC       libgnu_la-strndup.lo
strndup.c
C:\Program Files (x86)\Windows 
Kits\10\Include\10.0.10240.0\ucrt\corecrt_malloc.h(85): error C2375: 'free': 
redefinition; different linkage
.\string.h(644): note: see declaration of 'free'

This patch fixes it.


2022-05-15  Bruno Haible  <bruno@clisp.org>

        string, wchar: Fix compilation error on MSVC (regression 2021-09-07).
        * lib/string.in.h (free): For MSVC, add the dllimport specification if
        the MSVC headers have it.
        * lib/wchar.in.h (free): Likewise.

diff --git a/lib/string.in.h b/lib/string.in.h
index 33160b2525..3996da9fcb 100644
--- a/lib/string.in.h
+++ b/lib/string.in.h
@@ -122,8 +122,12 @@ _GL_EXTERN_C void rpl_free (void *);
 #  undef _GL_ATTRIBUTE_DEALLOC_FREE
 #  define _GL_ATTRIBUTE_DEALLOC_FREE _GL_ATTRIBUTE_DEALLOC (rpl_free, 1)
 # else
-#  if defined _MSC_VER
-_GL_EXTERN_C void __cdecl free (void *);
+#  if defined _MSC_VER && !defined free
+_GL_EXTERN_C
+#   if defined _DLL
+     __declspec (dllimport)
+#   endif
+     void __cdecl free (void *);
 #  else
 #   if defined __cplusplus && (__GLIBC__ + (__GLIBC_MINOR__ >= 14) > 2)
 _GL_EXTERN_C void free (void *) throw ();
@@ -133,8 +137,12 @@ _GL_EXTERN_C void free (void *);
 #  endif
 # endif
 #else
-# if defined _MSC_VER
-_GL_EXTERN_C void __cdecl free (void *);
+# if defined _MSC_VER && !defined free
+_GL_EXTERN_C
+#   if defined _DLL
+     __declspec (dllimport)
+#   endif
+     void __cdecl free (void *);
 # else
 #  if defined __cplusplus && (__GLIBC__ + (__GLIBC_MINOR__ >= 14) > 2)
 _GL_EXTERN_C void free (void *) throw ();
diff --git a/lib/wchar.in.h b/lib/wchar.in.h
index 0d5c0b7abe..835ddbe659 100644
--- a/lib/wchar.in.h
+++ b/lib/wchar.in.h
@@ -185,8 +185,12 @@ _GL_EXTERN_C void rpl_free (void *);
 #  undef _GL_ATTRIBUTE_DEALLOC_FREE
 #  define _GL_ATTRIBUTE_DEALLOC_FREE _GL_ATTRIBUTE_DEALLOC (rpl_free, 1)
 # else
-#  if defined _MSC_VER
-_GL_EXTERN_C void __cdecl free (void *);
+#  if defined _MSC_VER && !defined free
+_GL_EXTERN_C
+#   if defined _DLL
+     __declspec (dllimport)
+#   endif
+     void __cdecl free (void *);
 #  else
 #   if defined __cplusplus && (__GLIBC__ + (__GLIBC_MINOR__ >= 14) > 2)
 _GL_EXTERN_C void free (void *) throw ();
@@ -196,8 +200,12 @@ _GL_EXTERN_C void free (void *);
 #  endif
 # endif
 #else
-# if defined _MSC_VER
-_GL_EXTERN_C void __cdecl free (void *);
+# if defined _MSC_VER && !defined free
+_GL_EXTERN_C
+#   if defined _DLL
+     __declspec (dllimport)
+#   endif
+     void __cdecl free (void *);
 # else
 #  if defined __cplusplus && (__GLIBC__ + (__GLIBC_MINOR__ >= 14) > 2)
 _GL_EXTERN_C void free (void *) throw ();