Set default build profile to development
Updates the build to default to development builds if a git directory is found, otherwise require an explicit build profile to be set.
This means that by default, builds from tarballs must specify -Dprofile=…
with an appropriate value.
See also https://discourse.gnome.org/t/rfc-requiring-a-profile-to-be-specified-when-building-geary/4405/2