First thing I'd do is navigate to /usr/local/share/applications & see if there is a geany.desktop
If so then just drag onto the launcher.
If not then would just a minute or so to set up. Your source has a 'geany.desktop.in', remove the .in & it's a perfectly good geany.desktop
Edit: the geany.desktop.in is a good template for your .desktop but needs some minor editing before copying or after. Lines 4,5,6 start with a _ that should be removed.
Ex. of edited geany.desktop (or edited geany.desktop.in
GenericName=Integrated Development Environment
Comment=A fast and lightweight IDE using GTK2
The source also has some geany icons in it, take the geany.png from geany-0.21/icons/48x48 & put somewhere like Pictures or Documents
Then copy the geany.desktop.in file sitting in the source folder to ~/.local/share/applications/geany.desktop
Ex. - if it was in Downloads/geany-0.21
cp Downloads/geany-0.21/geany.desktop.in ~/.local/share/applications/geany.desktop
Then just go there,
right click on geany .desktop > properties > click on the icon area & browse to your geany.png > open
Finally drag geany.desktop onto your launcher
one advantage you'll see with a more 'proper' .desktop is it has a MimeTypes= line. Open up the .desktop in a text editor & take a look.
You can, if you want, now drag & drop any of those mimetypes on to the geany icon in your launcher & they'll open up in geany