setup_paths() sets app->datadir to $prefix/win32. With mingw-via-autotools,
it installed data to $prefix/share/geany (like on Linux). With this change
data is installed to $prefix/data ($datarootdir is not changed).

This fixes geany startup after make install with autotools.

Linux:
pkgdatadir = ${datarootdir}/geany
GEANY_DATA_DIR = /path/to/prefix/share/geany

Win32:
pkgdatadir = ${prefix}/data
GEANY_DATA_DIR = /path/to/prefix/data