Generate Makefile.silecs which can then be included in Makefile.specific...

Generate Makefile.silecs which can then be included in Makefile.specific instead f manually specifying the compiler flags. This addresses #80 (closed).

I am not sure if you also had in mind to automatically update the Makefile.specific or if we still let the user do it. For now I added a comment to the generated file that the user should include the generated makefile in the specific.

For newer releases also these instructions will need to be updated. https://www-acc.gsi.de/wiki/Frontend/Silecs_2_4_x_CodeSnippets#Dependency_in_Makefile.specific