pidgin/pidgin

Use LT_LIB_M to determine if we need to link to the math library and link Pidgin to the X11 libraries

Gentoo ran into an issue with the gold linker without -lm. The original patch
and work came from Justin Lechner. https://bugs.gentoo.org/386079

Gentoo also ran into an issue with X11_LIBS not being added to pidgin. This
should already be implicit everywhere except for homebrew which patches X11
out of their build. Original patch/work came from and. https://bugs.gentoo.org/500762

Testing Done:
Installed and ran locally.

Reviewed at https://reviews.imfreedom.org/r/608/
/** @page core-signals Core Signals
@signals
@signal quitting
@signal uri-handler
@endsignals
@see core.h
<hr>
@signaldef quitting
@signalproto
void (*quitting)();
@endsignalproto
@signaldesc
Emitted when libpurple is quitting.
@endsignaldef
@signaldef uri-handler
@signalproto
gboolean (*uri_handler)(const gchar *proto, const gchar *cmd, GHashTable *params);
@endsignalproto
@signaldesc
Emitted when handling a registered URI.
@param proto The protocol of the URI.
@param cmd The 'command' of the URI.
@param params Any key/value parameters from the URI.
@endsignaldef
*/
// vim: syntax=c.doxygen tw=75 et