emulators/uae: the port had been improved (+)

• Unbreak the build on modern FreeBSD versions by avoiding injecting duplicate symbols in the object files
• Drop MAKE_JOBS_UNSAFE: it was added back in 2013 in the commit 2a5c8652e56c1 to fix the build error:
  
  crt1.c:(.text+0xa8): undefined reference to `main' gmake[2]: *** [build68k] Error 1
  
  which no longer manifests itself

PR: 256577