build: pass -fcommon when building genisoimage

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.

Author: fanquake

depends/packages/native_cdrkit.mk

@@ -9,8 +9,10 @@ define $(package)_preprocess_cmds
   patch -p1 < $($(package)_patch_dir)/cdrkit-deterministic.patch
 endef
 
+# Starting with 10.1, GCC defaults to -fno-common, resulting in linking errors.
+# Pass -fcommon to retain the legacy behaviour.
 define $(package)_config_cmds
-  cmake -DCMAKE_INSTALL_PREFIX=$(build_prefix)
+  cmake -DCMAKE_INSTALL_PREFIX=$(build_prefix) -DCMAKE_C_FLAGS="-fcommon"
 endef
 
 define $(package)_build_cmds