LLVM needs _GNU_SOURCE defined, so let the SConscript handle that for the regex backend and BSD compat.

This BSD call will have to be implemented on windows. Right now
we have a stub that just exits the process.

NEWS:
* Switching endianness mid-byte no longer potentially re-reads bytes.
* bit_offset now consistently refers to the number of bits already
  read.
* HParsedTokens now have a bit_length field; this is a size_t.  This
  may be removed for memory reasons.

The bit writer has not yet been updated to match; the result of
switching bit writer endianness in the middle of a byte remains
undefined.

Go, and likely other language bindings will, need access to create
new combinators. This functionality is currently located
src/parsers/parser_internal.h. Install this header to the system with
dependent headers until a better solution can be found.

Create a pkg-config file and install it to $prefix/lib/pkgconfig