[skip ci] Solaris getexecname() returns argv[0]

`argv[0]`/`getexexname()` are not always absolute paths by default and can be modified to anything the developer wants them to be.

Consider using `readSymLink("/proc/self/path/a.out")` instead and `getexecname()` as the fallback, since the symlink will always be the correct absolute path (unless /proc is ot mounted, but it is by default on Solaris and Illumos platforms).

(cherry picked from commit 4f5e9fd5bda0d58c2907f75fa4ea0c92c3a2525f)

src/filesystem/unix/SDL_sysfilesystem.c

@@ -208,16 +208,6 @@ SDL_GetBasePath(void)
         SDL_free(cmdline);
     }
 #endif
-#if defined(__SOLARIS__)
-    const char *path = getexecname();
-    if ((path != NULL) && (path[0] == '/')) { /* must be absolute path... */
-        retval = SDL_strdup(path);
-        if (!retval) {
-            SDL_OutOfMemory();
-            return NULL;
-        }
-    }
-#endif
 
     /* is a Linux-style /proc filesystem available? */
     if (!retval && (access("/proc", F_OK) == 0)) {
@@ -228,6 +218,8 @@ SDL_GetBasePath(void)
         retval = readSymLink("/proc/curproc/file");
 #elif defined(__NETBSD__)
         retval = readSymLink("/proc/curproc/exe");
+#elif defined(__SOLARIS__)
+        retval = readSymLink("/proc/self/path/a.out");
 #elif defined(__QNXNTO__)
         retval = SDL_LoadFile("/proc/self/exefile", NULL);
 #else
@@ -245,6 +237,18 @@ SDL_GetBasePath(void)
 #endif
     }
 
+#if defined(__SOLARIS__)  /* try this as a fallback if /proc didn't pan out */
+    if (!retval) {
+        const char *path = getexecname();
+        if ((path != NULL) && (path[0] == '/')) { /* must be absolute path... */
+            retval = SDL_strdup(path);
+            if (!retval) {
+                SDL_OutOfMemory();
+                return NULL;
+            }
+        }
+#endif
+
     /* If we had access to argv[0] here, we could check it for a path,
         or troll through $PATH looking for it, too. */