Minor fixes:
 - CoRoutine2 selector requires that a GCC v5 compiler or better is selected
   - coroutine2 requires C++14
 - The Makefile was not alowing the libraries to be compiled with both
   statically and shared, at the same time. There are now two seperate options,
   allowing to select which version is wanted.
 - The Makefile was also not allowing to compile both single thread and multi-
   thread versions. Again, two seperate options now exist.
 - There is also the option to build another set of libraries with debug support
   which is good for development.
 - These options are important for those who whish to build an OpenWRT SDK.

Signed-off-by: Carlos M. Ferreira <carlosmf.pt@gmail.com>