Integrate autoconf build system to 2.4 RC

Autoconf is the new default build system for GENESIS, but the
legacy Makefiles are kept for backwards compatibility

- Makefile.BASE works for both legacy and autoconf build system
- Endianness detection added
- AC: INSTALLDIR now uses --prefix option from configure script
- Fix: netcdf compilation error on MacOS X due to ptrdiff_t
Merge pull request #17 from physicalist/master

Integrate autoconf build system to 2.4 RC