Use the user specified target .NET framework for Managed C++ VS projects.

--HG--
branch : cpp-framework
Set CXX to clang++ when toolset is clang

Without this, clang (instead of C++) is used even
when compiling C++ code.
Fix clang test case

Test asserts that CXX is set to clang++ (and not clang)
Handle case for optimize "Off"

Makes gcc/clang use '-O0' when optimize "Off" is set
Merged in nitingupta910/premake-dev (pull request #81)

Fixes for gcc/clang
Merged in triton/premake-cs-fixes/cpp-framework (pull request #79)

Use the user specified target .NET framework for Managed C++ VS projects.