When compiling with gcc -Wundef, we get a compilation error of:
.../stlport/stl/_bitset.h:723:34: "_STLP_MSVC" is not defined
Looking at that code:
# if !(defined (_STLP_MSVC) || (_STLP_MSVC < 1300)) && \ !(defined(__SUNPRO_CC) || (__SUNPRO_CC < 0x500))
that's clearly wrong - if _STLP_MSVC is not defined, it'll go on and test it for being less than 1300 which is going to cause a compilation error if -Wundef is in effect. Similarly the test on __SUNPROC_CC in the following line.