Set LD_LIBRARY_PATH in unix run environments.

Similarly to how PATH is set up for Windows. The background is that
without it plugin loading can pull in incompatible Qt libraries if the
binary is compiled with RUNPATH instead of RPATH.

Reviewed-by: Daniel Teske

src/libs/utils/environment.h

@@ -85,6 +85,8 @@ public:
     void appendOrSetPath(const QString &value);
     void prependOrSetPath(const QString &value);
 
+    void prependOrSetLibrarySearchPath(const QString &value);
+
     void clear();
     int size() const;