microprofile: unbreak on POSIX systems

In file included from src/common/microprofile.cpp:7:
In file included from src/./common/microprofile.h:23:
externals/microprofile/microprofile.h:830:5: error: use of undeclared identifier 'MP_BREAK'
    MP_ASSERT(t == nBegin);
    ^
externals/microprofile/microprofile.h:238:34: note: expanded from macro 'MP_ASSERT'
                                 ^
externals/microprofile/microprofile.h:831:5: error: use of undeclared identifier 'MP_BREAK'
    MP_ASSERT(nTimerIndex == (nToken&0x3fff));
    ^
externals/microprofile/microprofile.h:238:34: note: expanded from macro 'MP_ASSERT'
                                 ^
[...]