Modernize build

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 -fcommon compiler 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.
• Avoid mkdir error by using -p option.