Thanks to Olesya Gerasimenko <gammaray@basealt.ru> for the work! :D

plain ./configure without --disable-png needs libpng-dev

Added libpng.a as a dependency
Updated SDKROOT to macosx instead of referencing a specific SDK version

Rename OSX to macOS and provide missing installation instructions for building a Pidgin plugin under macOS

Thanks to @halftux
Closes #501.

Thanks to Eduardo Trápani <etrapani@gmail.com>
for fixing the nasty UTF-8 bug that Transifex apparently introduced!

Credits to this oneliner:
$ grep -Prn '(?<!along with this program; )(?<!along with this library; )(?<![A-Za-z_#])if[^{]+$' *.c | grep -Fv '//'

# Conflicts:
#	tgp-msg.c

I love you too, GitHub Flavored Mardown.

Slightly nicer wording.

As requested in the group chat, adding a section to FAQ to deal with users on lesser-used distributions and/or users new to linux as a whole.