Merge #19553: build: pass -fcommon when building genisoimage

2a701a1c42 build: pass -fcommon when building genisoimage (fanquake)

Pull request description:

  Starting with the 10.1 release, GCC [defaults to -fno-common](https://gcc.gnu.org/gcc-10/porting_to.html). 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, just pass -fcommon to preserve
  the legacy GCC behaviour.

  Noticed [while testing #19530](https://github.com/bitcoin/bitcoin/pull/19530#issuecomment-659802512).

ACKs for top commit:
  laanwj:
    ACK 2a701a1c42
  hebasto:
    ACK 2a701a1c42, tested on Fedora 32 (x86_64, GCC 10.1.1):

Tree-SHA512: 0e6dd1b4c7c3f5b6997616da27a7ea54f757c81677766ee6813c776356527787a32b959f834e699fee9798553b13bcb142469f442c5e7a04f810f2c06e3a9505

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