This compiler is not supported yet, or maybe we should say this differently: this compiler has a lot of quirks which makes it reject the library at too many places for us to find workarounds. It has a number of problems which seem to happen randomly -- trying to cut down on the code makes them go away, so that it is hard to come up with short testcases.
Roughly estimated, there may be about 5 to 10 bugs in the compiler that prevent it to compile the library. I think one can say that it is close, but not there yet.
However, the compiler is detected by the configuration machinery, so testing newer versions of the compiler should be simple once they are released. Borland C++ 5.7 is part of both Borland's Windows C++ compiler kit as well as the Kylix3 environment for Linux.