[PATCH setup] Avoid stringop-overflow warning with gcc8

[Jon Turney <jon.turney@dronecode.org.uk>]

desktop.cc: In function 'void start_menu(const string&, const string&, const string&, const string&)':
desktop.cc:110:11: error: 'char* strncat(char*, const char*, size_t)' specified bound 260 equals destination size [-Werror=stringop-overflow=]

I think strlcat() was meant here, which MinGW doesn't have.  In it's
absence, open-code it's equivalent.

(SHGetSpecialFolderLocation() returns a pathname of length at most MAX_PATH,
and make_link() is limited to accepting a pathname of length MAX_PATH, so we
want to append our folder name, while truncating the result to MAX_PATH.)
---
 desktop.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/desktop.cc b/desktop.cc
index 927c02f..d003e91 100644
--- a/desktop.cc
+++ b/desktop.cc
@@ -107,7 +107,8 @@ start_menu (const std::string& title, const std::string& target,
 			      issystem ? CSIDL_COMMON_PROGRAMS :
 			      CSIDL_PROGRAMS, &id);
   SHGetPathFromIDList (id, path);
-  strncat (path, "/Cygwin", MAX_PATH);
+  strncat (path, "/Cygwin", MAX_PATH - strlen(path));
+  path[MAX_PATH-1] = 0;
   LogBabblePrintf ("Program directory for program link: %s", path);
   make_link (path, title, target, arg, iconpath);
 }
-- 
2.17.0