Fix compilation on GCC v10 and higher

GCC compiler v10 and higher throws exception "Multiple definitions of ... First defined here". He we telling compiler to ignore this error, otherwise the software will not compile.

Author: mickolaua

Makefile

@@ -27,7 +27,7 @@ LIBDIR        =  ../../cfitsio/lib
 
 # standard usage
 # recently added -std=c99 after a bug report
-COPTS = -funroll-loops -O3 -ansi -std=c99 -pedantic-errors -Wall -I$(CFITSIOINCDIR) -D_GNU_SOURCE
+COPTS = -funroll-loops -fcommon -O3 -ansi -std=c99 -pedantic-errors -Wall -I$(CFITSIOINCDIR) -D_GNU_SOURCE
 LIBS  = -L$(LIBDIR) -lm -lcfitsio
 
 # compiler