Changed paths.bin to respect BINDIR

The compiled-in path to bin dir was not correct; it made the
common assumption that `$(prefix)` == `$(prefix)/bin` which is
usually true, but not in all cases.

1. [machine specific files](http://www.gnu.org/prep/standards/html_node/Directory-Variables.html#index-exec_005fprefix)
   should use `$(exec_prefix)` (`@exec_prefix@` in Makefile.in)
   instead of the usual `$(prefix)`.

2. [executable programs](http://www.gnu.org/prep/standards/html_node/Directory-Variables.html#index-bindir)
   should use the more-specific `$(bindir)` (`@bindir@` in Makefile.in)
   instead of assuming `$(exec_prefix)/bin` as this allows the
   executable install location to be changed with the
   `--bindir=_foo_` option to `./configure`.

Fortunately, the makefile is mostly correct and *installs* the
executable in the correct location. The problem shows up later
such as during tab-completion, when fish_pager is run - the
compiled-in path it uses was the incorrect `$(prefix)/bin`
resulting in an "Unknown command" error, which only required
this small fix.

fish.cpp

@@ -202,7 +202,7 @@ static struct config_paths_t determine_config_directory_paths(const char *argv0)
         paths.data = L"" DATADIR "/fish";
         paths.sysconf = L"" SYSCONFDIR "/fish";
         paths.doc = L"" DATADIR "/doc/fish";
-        paths.bin = L"" PREFIX "/bin";
+        paths.bin = L"" BINDIR;
 
         done = true;
     }