Insert additional -fcommon C flag to obtain the same behavior of previous versions of GCC compilers, because
now, by default GCC 10 compiler has the -fno-common flag
that give error when omitting extern when declaring
a global variable in a header file.
TODO: for the next MentOS release check the "extern"
keyword in global variable in header files.
Mirco De Marchi
05397f24c9