Fix several warnings in Windows build of type:
redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
Add the "subdir-objects" automake option to fix a warning issued by automake 1.14.
automake-1.14: warning: possible forward-incompatibility.
automake-1.14: At least a source file is in a subdirectory, but the 'subdir-objects'
automake-1.14: automake option hasn't been enabled.  For now, the corresponding output
automake-1.14: object file(s) will be placed in the top-level directory.  However,
automake-1.14: this behaviour will change in future Automake versions: they will
automake-1.14: unconditionally cause object files to be placed in the same subdirectory
automake-1.14: of the corresponding sources.
automake-1.14: You are advised to start using 'subdir-objects' option throughout your
automake-1.14: project, to avoid future incompatibilities.
Ignore generated files produced by previous commit.
Merge pull request #47 from genete/subdir-objects

Fix several configure and compile warnings.