The first problem is that the gcc option `-pthread' isn't documented for linux / x86. If you search http://gcc.gnu.org/onlinedocs/gcc-3.4.0/gcc/Option-Summary.html then "pthread" is found only among the RS/6000 and PowerPC options. It's therefore always some guesswork what you can expect from gcc if you compile with pthread support. In particular, it's not documented how you can *detect* pthread support.
IIUC POSIX does not (implicitly or explicitly) require any special compiler flags when you compile C code that uses a pthread implementation. I am not even sure whether it is strictly necessary to use the `-pthread' flag if you compile and link such code with gcc or whether it it is mainly a convenience option. (The situation may be different for C++ code if, e.g., the EH code is affected by the pthread support. But POSIX ignores C++, doesn't it?)
If I am correct then POSIX cannot require any means to detect *activated* pthread support. I rather think it is a QoI defect of gcc 3.4.
As far as POSIX is concerned then checking for _POSIX_THREADS in unistd.h is sufficient, and this probably is the case for C compilers, but as you say for C++ this is not the case. BTW _REENTRANT isn't standardised anywhere: it's just used informally by some Unix C/C++ compilers to indicate when thread safe code generation is turned on, which makes gcc's decission to define it in a header even more annoying. John.