Define NO_GSSAPI when -DPUTTY_GSSAPI=OFF

Previously it would only be defined if -DPUTTY_GSSAPI=DYNAMIC and libdl is
not found on the system. Setting -DPUTTY_GSSAPI=OFF on systems with glibc
2.33 and earlier causes a build failure, because libdl is not linked in.

Bug: https://bugs.gentoo.org/873355
--- a/cmake/platforms/unix.cmake
+++ b/cmake/platforms/unix.cmake
@@ -105,6 +105,8 @@
       "Could not find libdl -- cannot provide dynamic GSSAPI support")
     set(NO_GSSAPI ON)
   endif()
+else()
+  set(NO_GSSAPI ON)
 endif()
 
 if(PUTTY_GSSAPI STREQUAL STATIC)