Behave like STL.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fixes false negatives:

 http://foo/dav/example.ogg mismatches http://foo/dav/

.. because StringAfterPrefix() returns just "example.ogg", without
trailing slash (it existed, but was eaten already by the base
matcher).

New in clang 3.6.

Move code from Path::RelativeFS() and make it generic.