pidgin/pidgin

Don't allow libraries to include their own global header files.

Testing Done:
Compiled, ran unit tests, and built the docs

Reviewed at https://reviews.imfreedom.org/r/232/
subdir('libpurple')
subdir('protocols')
if ENABLE_GTK
subdir('pidgin')
else
pidgin_doc = []
endif
if enable_consoleui
subdir('finch')
else
finch_doc = []
endif
if meson.version().version_compare('>=0.52.0')
alias_target('doc', libpurple_doc, protocols_doc, pidgin_doc, finch_doc)
endif