No declaration of global variables in header files anymore

Current master version will not compile on UBUNTU 20+ using gcc --version 10+. The main issue is that there are lots of places of declaring global variables in headers and sometimes multiple times which the older compiler versions seemed to figure out. This fix addresses the suggestion in a previous merge request (in part) to compile with the -fcommon flag. The issue is more systemic than just a flag change and the code has been updated. Now compiles on UBUNTU 20+ with gcc --version 10.3.0 and clang and all older versions of gcc.