Closed requested to merge modernize_build into master
Update the build process for compatibility with newer systems, specifically Ubuntu 20.0.4. Changes:
- Force separate object files to share common variables by using the
-fcommoncompiler directive. GCC compilers before GCC 10 ignored this error, see https://gcc.gnu.org/gcc-10/porting_to.html.
- To avoid makefile warning, use pattern rule instead of obsoleted suffix rule in Makefile, see https://stackoverflow.com/questions/9233447/what-is-the-makefile-target-c-o-for.
mkdirerror by using