This follows the [recommendations from upstream](https://discourse.gnome.org/t/please-build-against-libsoup-3-by-default/10190/3). Note that this changes the option name from `soup3` to `soup2`.
Testing Done:
Compiled against libsoup3 and libsoup2.
Reviewed at https://reviews.imfreedom.org/r/2028/
# UPDATING VERSION NUMBERS FOR RELEASES
#
# The version number is:
# <major>.<minor>.<micro><extra>
#
# micro += 1
#
# If any functions have been added to libpurple, Pidgin, or Finch:
# micro = 0
# minor += 1
#
# If backwards compatibility has been broken in libpurple, Pidgin, or Finch:
# micro = 0
# minor = 0
# major += 1
# purple_soversion += 1
#
# extra should be similar to one of the following: