Compiling requires `-fcommon` flag on GCC v10+

[austintwang]

GCC 10 and newer defaults to -fno-common behavior, which results in multiple definition errors.
https://gcc.gnu.org/gcc-10/porting_to.html

Would be good to update the makefile or modify the header declarations to use extern

[austintwang]

A temporary workaround for building RASQUAL is to add -fcommon to the CFLAGS envvar.