MFH: r553443

audio/ardour6: fix build on GCC architectures

Undefine _POSIX_SOURCE to make macros in errno.h available:
/usr/local/lib/gcc9/include/c++/powerpc64-portbld-freebsd12.1/bits/error_constants.h:42:40: error: 'EAFNOSUPPORT' was not declared in this scope

42 |       address_family_not_supported =   EAFNOSUPPORT,
   |                                        ^~~~~~~~~~~~

/usr/local/lib/gcc9/include/c++/powerpc64-portbld-freebsd12.1/bits/error_constants.h:43:28: error: 'EADDRINUSE' was not declared in this scope

43 |       address_in_use =     EADDRINUSE,
   |                            ^~~~~~~~~~

Also tested to build with Clang.

Approved by: portmgr (fix build blanket)