Drop GCC -fcommon so that the new default -fno-common is used

Thanks to @greno4ka for the tip in
 https://github.com/mpereira/tty-solitaire/pull/58

Makefile

@@ -2,7 +2,7 @@ VERSION = 1.3.1
 
 CC     ?= gcc
 CFLAGS ?= -g
-CFLAGS += -W -Wall -pedantic -ansi -std=c99 -DVERSION=\"$(VERSION)\" -fcommon
+CFLAGS += -W -Wall -pedantic -ansi -std=c99 -DVERSION=\"$(VERSION)\"
 
 # The Ncurses library with wide character support is available as "lncurses"
 # under macOS.