mingw: try to create symlinks without elevated permissions

With Windows 10 Build 14972 in Developer Mode, a new flag is supported
by CreateSymbolicLink() to create symbolic links even when running
outside of an elevated session (which was previously required).

This new flag is called SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE and
has the numeric value 0x02.

Previous Windows 10 versions will not understand that flag and return an
ERROR_INVALID_PARAMETER, therefore we have to be careful to try passing
that flag only when the build number indicates that it is supported.

For more information about the new flag, see this blog post:
https://blogs.windows.com/buildingapps/2016/12/02/symlinks-windows-10/

This patch is loosely based on the patch submitted by Samuel D. Leslie
as #1184.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -365,6 +365,8 @@ static const wchar_t *make_relative_to(const wchar_t *path,
 	return out;
 }
 
+static DWORD symlink_file_flags = 0, symlink_directory_flags = 1;
+
 enum phantom_symlink_result {
 	PHANTOM_SYMLINK_RETRY,
 	PHANTOM_SYMLINK_DONE,
@@ -415,7 +417,8 @@ process_phantom_symlink(const wchar_t *wtarget, const wchar_t *wlink)
 		return PHANTOM_SYMLINK_DONE;
 
 	/* otherwise recreate the symlink with directory flag */
-	if (DeleteFileW(wlink) && CreateSymbolicLinkW(wlink, wtarget, 1))
+	if (DeleteFileW(wlink) &&
+	    CreateSymbolicLinkW(wlink, wtarget, symlink_directory_flags))
 		return PHANTOM_SYMLINK_DIRECTORY;
 
 	errno = err_win_to_posix(GetLastError());
@@ -2901,7 +2904,7 @@ int symlink(const char *target, const char *link)
 			wtarget[len] = '\\';
 
 	/* create file symlink */
-	if (!CreateSymbolicLinkW(wlink, wtarget, 0)) {
+	if (!CreateSymbolicLinkW(wlink, wtarget, symlink_file_flags)) {
 		errno = err_win_to_posix(GetLastError());
 		return -1;
 	}
@@ -3837,6 +3840,24 @@ static void maybe_redirect_std_handles(void)
 				  GENERIC_WRITE, FILE_FLAG_NO_BUFFERING);
 }
 
+static void adjust_symlink_flags(void)
+{
+	/*
+	 * Starting with Windows 10 Build 14972, symbolic links can be created
+	 * using CreateSymbolicLink() without elevation by passing the flag
+	 * SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE (0x02) as last
+	 * parameter, provided the Developer Mode has been enabled. Some
+	 * earlier Windows versions complain about this flag with an
+	 * ERROR_INVALID_PARAMETER, hence we have to test the build number
+	 * specifically.
+	 */
+	if (GetVersion() >= 14972 << 16) {
+		symlink_file_flags |= 2;
+		symlink_directory_flags |= 2;
+	}
+
+}
+
 #ifdef _MSC_VER
 #ifdef _DEBUG
 #include <crtdbg.h>
@@ -3871,6 +3892,7 @@ int wmain(int argc, const wchar_t **wargv)
 #endif
 
 	maybe_redirect_std_handles();
+	adjust_symlink_flags();
 	fsync_object_files = 1;
 
 	/* determine size of argv and environ conversion buffer */