Utility: fix Tweakable to work correctly with __FILE__ being absolute.

This is what I get for testing *exclusively* with Ninja.

doc/corrade-changelog.dox

@@ -136,6 +136,9 @@ namespace Corrade {
     [mosra/corrade#51](https://github.com/mosra/corrade/issues/51))
 -   Fixed @ref CORRADE_COMPARE_WITH() to be callable with expressions such as
     @cpp *comparator @ce
+-   Fixed @ref Utility::Tweakable to work correctly with buildsystems using
+    absolute paths for `__FILE__` macros (such as Makefiles, Ninja is using
+    relative paths). See [mosra/corrade#61](https://github.com/mosra/corrade/issues/61).
 
 @subsection corrade-changelog-latest-deprecated Deprecated APIs
 

src/Corrade/Utility/Tweakable.cpp

@@ -107,10 +107,11 @@ std::pair<bool, void*> Tweakable::registerVariable(const char* const file, const
     if(found == _data->files.end()) {
         /* Strip the directory prefix from the file. If that means the filename
            would then start with a slash, strip that too so Directory::join()
-           works correctly. */
+           works correctly -- but don't do that in case the directory prefix
+           was empty, in that case the file path was absolute. */
         /** @todo maybe some Directory utility for this? */
         std::string stripped = String::stripPrefix(Directory::fromNativeSeparators(file), _data->prefix);
-        if(!stripped.empty() && stripped.front() == '/')
+        if(!_data->prefix.empty() && !stripped.empty() && stripped.front() == '/')
             stripped.erase(0, 1);
 
         const std::string watchPath = Directory::join(_data->replace, stripped);