modules: properly save CMAKE_FIND_LIBRARY_PREFIXES in FindMagnum.cmake.

On Emscripten this was equal to ";somebogusprefix" (as per the
toolchain), which got expanded into just somebogusprefix, losing the
empty string. Saving it back to CMAKE_FIND_LIBRARY_PREFIXES would cause
all subsequent find_library() calls to fail, because no library is named
somebogusprefixMagnum*.bc.

Thinking about this, the "somebogusprefix" was probably added "back
then" as a workaround to CMake complaining that I'm messing up in the
Find module. So I guess I can remove that as well.

modules/FindMagnum.cmake

@@ -455,8 +455,8 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
             # Dynamic plugins don't have any prefix (e.g. `lib` on Linux),
             # search with empty prefix and then reset that back so we don't
             # accidentaly break something else
-            set(_tmp_prefixes ${CMAKE_FIND_LIBRARY_PREFIXES})
-            set(CMAKE_FIND_LIBRARY_PREFIXES ${CMAKE_FIND_LIBRARY_PREFIXES} "")
+            set(_tmp_prefixes "${CMAKE_FIND_LIBRARY_PREFIXES}")
+            set(CMAKE_FIND_LIBRARY_PREFIXES "${CMAKE_FIND_LIBRARY_PREFIXES};")
 
             # Try to find both debug and release version. Dynamic and static
             # debug libraries are in different places.