Support for musl/Alpine Linux

I've been working on making WebkitFLTK build on Alpine Linux.
This is enough to get up to the final link; *however*, the final link does not work because Alpine does not provide _static_ icu/harfbuzz/cairo/... libraries.

Changes:
-I noted the build dependencies that I had to install in README.asciidoc
-DisallowCType.h was problematic on Linux with a generic libstdc++, like musl uses; I added non-glibc Linux to the list of of excluded platforms.
-Assertions.cpp checked for Linux rather than glibc when deciding whether to use backtrace(), and the uclibc path didn't work either; I guessed that OS(LINUX) meant glibc||uclibc originally.
-A couple occurrences of isnan() caused errors; g++ suggested std::isnan().
-In dirlisting.cpp, g++ (4.8.3) complained about comp(const void *, const void *) having a different type from comp(const struct dirent **, const struct dirent **).