Starting with the 10.1 release, GCC defaults to -fno-common. This causes
linking issues when building genisoimage:
```bash
[ 98%] Building C object
genisoimage/CMakeFiles/genisoimage.dir/checksum.o
[100%] Linking C executable genisoimage
/usr/bin/ld: CMakeFiles/genisoimage.dir/apple.o:(.bss+0x0): multiple
definition of `outfile';
CMakeFiles/genisoimage.dir/genisoimage.o:(.bss+0x0): first defined here
/usr/bin/ld: CMakeFiles/genisoimage.dir/boot.o:(.bss+0x0): multiple
definition of `outfile';
CMakeFiles/genisoimage.dir/genisoimage.o:(.bss+0x0): first defined here
/usr/bin/ld: CMakeFiles/genisoimage.dir/desktop.o:(.bss+0x0): multiple
definition of `outfile';
CMakeFiles/genisoimage.dir/genisoimage.o:(.bss+0x0): first defined here
/usr/bin/ld: CMakeFiles/genisoimage.dir/dvd_file.o:(.bss+0x0): multiple
definition of `outfile';
CMakeFiles/genisoimage.dir/genisoimage.o:(.bss+0x0): first defined here
```
Rather than patching genisoimage further, pass -fcommon to preserve the
legacy GCC behaviour.
Backport of core PR19553.
Fixes the OSX depends build on Arch.