patch 8.0.0450: v:progpath is not reliably set

Problem:    v:progpath is not reliably set.
Solution:   Read /proc/self/exe if possible. (idea by Michal Grochmal)
            Also fixes missing #if.

src/config.h.in

@@ -446,6 +446,9 @@
 /* Define if fcntl()'s F_SETFD command knows about FD_CLOEXEC */
 #undef HAVE_FD_CLOEXEC
 
+/* Define if /proc/self/exe can be read */
+#undef HAVE_PROC_SELF_EXE
+
 /* Define if you want Cygwin to use the WIN32 clipboard, not compatible with X11*/
 #undef FEAT_CYGWIN_WIN32_CLIPBOARD
 

src/main.c

@@ -3539,16 +3539,27 @@ set_progpath(char_u *argv0)
 {
     char_u *val = argv0;
 
+# ifdef HAVE_PROC_SELF_EXE
+    char    buf[PATH_MAX + 1];
+    ssize_t len;
+
+    len = readlink("/proc/self/exe", buf, PATH_MAX);
+    if (len > 0)
+    {
+	buf[len] = NUL;
+	val = (char_u *)buf;
+    }
+# else
     /* A relative path containing a "/" will become invalid when using ":cd",
      * turn it into a full path.
      * On MS-Windows "vim" should be expanded to "vim.exe", thus always do
      * this. */
-# ifdef WIN32
+#  ifdef WIN32
     char_u *path = NULL;
 
     if (mch_can_exe(argv0, &path, FALSE) && path != NULL)
 	val = path;
-# else
+#  else
     char_u buf[MAXPATHL];
 
     if (!mch_isFullName(argv0))

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    450,
 /**/
     449,
 /**/