The <cxxx> headers for the C library are not under our control, so we
can't prevent them from including <unistd.h>. Change the PR 49745 test
to only include the C++ library headers, not the <cxxx> ones.
To ensure <bits/stdc++.h> isn't included automatically we need to use
no_pch to disable PCH.
libstdc++-v3/ChangeLog:
PR libstdc++/100117
* testsuite/17_intro/headers/c++1998/49745.cc: Explicitly list
all C++ headers instead of including <bits/stdc++.h>
77963796ae