Fix atexit() linking error
Recent versions of glibc expose atexit() as a hidden symbol in libc_nonshared.a, causing linking failures when building OpenDoors as a shared library.
This fix switches from using $(LD) directly to $(CC) (gcc) to invoke the linker,
and adds -Wl,-Bsymbolic
to force internal resolution of symbols, resolving the
"hidden symbol atexit
... is referenced by DSO" error.
Also adds -lc
explicitly to ensure glibc is linked properly.
Tested on Debian 12 and Ubuntu 24.04.