Switch to the Autotools

The autotools provide a more standard way to generate binaries on linux but also using mingw32. The dependencies are checked using pkg-config which will ease maintenance.

To compile on Linux:
 $ ./configure
 $ make
 $ make install

To cross-compile to Windows:
 $ mingw32-configure
 $ make

A reqflow-1.3.0-setup.exe will be generated.