* Make math library optional.
As the comment says, it should be optional on Windows and Haiku, but the check defaults to required.
* Don't make `unistd.h` required.
* Disable transparency plugin build as it's very busted.
* Add missing headers to fix compile on Windows.
* Use a fixed pref key for test plugin loading instead of `TEST_DATA_DIR`.
The latter is an absolute path, and thus unsuitable as a preference key, since it will not start with `/` on Windows.
* Add a missing pref initialization on Windows.
Testing Done:
Compiled and run tests on Windows.
Reviewed at https://reviews.imfreedom.org/r/308/
# 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: