Make Source/WTF/wtf build on Linux other than glibc/uclibc.

* Assertions.cpp relied on backtrace/dlfcn on Linux.
  musl doesn't provide these, so look for glibc/uclibc rather than linux.

* DisallowCType.h would break a generic libstdc++, since ctype.h is
  used internally.
Fix compilation error on isnan()

s/\(isnan\)/std::\1/ is what I used, but I'm not sure if just including
another header might be better.