build: Require notify-sharp-3.0 and clean up configure checks

Now that notify-sharp has a 3.0 release that depends on GTK 3, we can
require the proper pkg-config module.

Remove notify-sharp.m4 file, and the useless call to the macro that was
defined in it.

Also remove some unneeded variables in build.environment.mk.
Update README.GNOME3.md
Update README.GNOME3.md
Update README.GNOME3.md
Update README.GNOME3.md
Update README.GNOME3.md
Update README.GNOME3.md
Update README.GNOME3.md
git linux: Check user's local bin folder for possibly newer git version. Closes #1425
Merge branch 'master' of https://github.com/hbons/SparkleShare