Updated autoconf/automake files to support cross-compiling.
It also generates a shared library by default, including a DLL when compiling for Windows.
I should not have been calling `pkg-config` in the Makefile.am files to get the GTK3 cflags.
Building gtkdatabox now works with mingw32 and mingw64 on fedora linux.
Adding -no-undefined and AC_LIBTOOL_WIN32_DLL enables DLL building (but haven't tested if DLL is usable).
Note: autoconf now defaults to building the noinstall example programs dynamically linked, with script files in examples, actual binaries in examples/.libs. Can call configure --disable-shared to avoid this and go back to original behavior (this is an autoconf thing, but need to know this in order to use gdb on the example programs)