Massage build of linux-dwarffortress

• change back from c6 to the default (c7 now)
• don't install the bundled libc++, because it requires glibc 2.18 which c7 doesn't have
• avoid a runtime error message about sound-not-loading by switching off sound in the shipped config files. These are copied to the user's ~/.dwarffortress directory and can be edited there if needed.

PR: 240063