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.

Fixes writing to Context.getCacheDir().

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.

Prepare to add more modes.

The Linux feature allows writing new files to an invisible file, and
then replace the old file.  This preserves the old file if we get
interrupted by some event.

Necessary for "final" on gcc 4.6.

To replace g_strdup().

The last piece to allow -ffast-math.