Remove custom deduction guides for std::unique_ptr in C++17 mode.
Issue: FreeBSD GCC 13.3 correctly rejects adding deduction guides
to namespace std, which violates C++ standard [namespace.std]:
"The behavior of a C++ program is undefined if it adds declarations
or definitions to namespace std or to a namespace within namespace std."
Root cause: The code attempted to add custom deduction guides for
std::unique_ptr in namespace std when compiling in C++17 mode.
This is not allowed by the C++ standard.
Solution: Remove the custom deduction guides for C++17 and later,
as the C++17 standard library already provides deduction guides
for std::unique_ptr (added in C++17 via P0433R2).
The custom deduction guide wrappers in the else branch (for C++14
and earlier) are kept as they provide helper functions, not actual
deduction guides in namespace std.
Tested-on: FreeBSD 15 with GCC 13.3
Fixes: Compilation error 'deduction guide must be declared in the
same scope as template std::unique_ptr'
- Add IPC_OS_FREEBSD_ platform detection macro
- Enable FreeBSD to use POSIX pthread implementation (shared with QNX)
- Update all conditional compilation directives to include FreeBSD
- Update README to reflect FreeBSD platform support
FreeBSD uses the existing POSIX implementation which supports:
- Process-shared mutexes (PTHREAD_PROCESS_SHARED)
- Robust mutexes (PTHREAD_MUTEX_ROBUST)
- Timed lock operations
- POSIX shared memory
This is a minimal change that reuses the mature POSIX implementation
already proven by QNX platform support.
