Improved error handling, Windows fixes, Autotool fixes

• Previously, when there was an error in MMDB_open(), errno would
  generally be overwritten during cleanup, preventing a useful value from
  being returned to the caller. This was changed so that the errno value
  from the function call that caused the error is restored before returning to
  the caller. In particular, this is important for MMDB_IO_ERROR errors as
  checking errno is often the only way to determine what actually failed.
• If mmap() fails due to running out of memory space, an
  MMDB_OUT_OF_MEMORY_ERROR is now returned from MMDB_open rather than an
  MMDB_IO_ERROR.
• On Windows, the CreateFileMappingA() handle was not properly closed if
  opening the database succeeded. Fixed by Bly Hostetler. GitHub #75 & #76.
• On Windows, we were not checking the return value of CreateFileMappingA()
  properly for errors. Fixed by Bly Hotetler. GitHub #78.
• Several warnings from Clang's scan-build were fixed. GitHub #86.
• All headers are now installed in $(includedir). GitHub #89.
• We no longer install maxminddb-compat-util.h. This header was intended for
  internal use only.