libprom: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

    ld: CMakeFiles/prom.dir/src/prom_process_stat.c.o:(.bss+0x0): multiple definition of
      `prom_process_start_time_seconds'; CMakeFiles/prom.dir/src/prom_collector.c.o:(.bss+0x0): first defined here

pkgs/development/libraries/prometheus-client-c/default.nix

@@ -43,6 +43,13 @@ let
         )
       ];
 
+      # Workaround build failure on -fno-common toolchains like upstream
+      # gcc-10. Otherwise build fails as:
+      #   ld: CMakeFiles/prom.dir/src/prom_process_stat.c.o:(.bss+0x0): multiple definition of
+      #     `prom_process_start_time_seconds'; CMakeFiles/prom.dir/src/prom_collector.c.o:(.bss+0x0): first defined here
+      # Should be fixed in 1.2.0 and later: https://github.com/digitalocean/prometheus-client-c/pull/25
+      NIX_CFLAGS_COMPILE = "-fcommon";
+
       preConfigure = ''
         cd ${subdir}
       '';