Without the change build agaist -fno-common toolchains like upstream
gcc-10 fails as:
$ nix build -L --impure --expr 'with import ./. {}; rox-filer.overrideAttrs (oa: { NIX_CFLAGS_COMPILE = (oa.NIX_CFLAGS_COMPILE or "") + "-fno-common"; })'
...
ld: session.o:/build/rox-filer-2.11/ROX-Filer/build/../src/session.h:13:
multiple definition of `session_auto_respawn';
main.o:/build/rox-filer-2.11/ROX-Filer/build/../src/session.h:13:
first defined here
While at it normalized local patch against top-level source tree
to ease application of upstream patches as is.
