Move some add_executable() calls to top-level CMakeLists.

Now that the main source file of Plink in each platform directory has
the same name, we can put centralise the main definition of the
program in the main CMakeLists.txt, and in the platform directory,
just add the few extra modules needed to clear up platform-specific
details.

The same goes for psocks. And PSCP and PSFTP could have been moved to
the top level already - I just hadn't done it in the initial setup.

cmake/platforms/unix.cmake

@@ -1,5 +1,3 @@
-set(PLATFORM_SUBDIRS charset unix)
-
 set(PUTTY_GSSAPI DYNAMIC
   CACHE STRING "Build PuTTY with dynamically or statically linked \
 Kerberos / GSSAPI support, if possible")
@@ -80,6 +78,8 @@ add_optional_system_lib(m pow)
 add_optional_system_lib(rt clock_gettime)
 add_optional_system_lib(xnet socket)
 
+set(extra_dirs charset)
+
 if(PUTTY_GSSAPI STREQUAL DYNAMIC)
   add_optional_system_lib(dl dlopen)
   if(HAVE_NO_LIBdl)

cmake/platforms/windows.cmake

@@ -1,5 +1,3 @@
-set(PLATFORM_SUBDIRS windows)
-
 set(PUTTY_MINEFIELD OFF
   CACHE BOOL "Build PuTTY with its built-in memory debugger 'Minefield'")
 set(PUTTY_GSSAPI ON