GCC 10 incompatibility (missing extern for global variables in headers)

[robert-scheck]

Since Fedora is using GCC 10, building Partclone fails like this:

/usr/bin/ld: partclone.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:83: multiple definition of `rescue_write_size'; info.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:83: first defined here
/usr/bin/ld: partclone.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:82: multiple definition of `EXECNAME'; info.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:82: first defined here
/usr/bin/ld: checksum.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:83: multiple definition of `rescue_write_size'; info.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:83: first defined here
/usr/bin/ld: checksum.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:82: multiple definition of `EXECNAME'; info.o:/builddir/build/BUILD/partclone-0.3.12/src/partclone.h:82: first defined here

and

/usr/bin/ld: partclone_xfs-xfsclone.o:/builddir/build/BUILD/partclone-0.3.12/src/xfsclone.c:27: multiple definition of `prog'; partclone_xfs-main.o:/builddir/build/BUILD/partclone-0.3.12/src/main.c:42: first defined here

https://gcc.gnu.org/gcc-10/porting_to.html describes how these mistakes in C code can be fixed.