pidgin: use pkg-config instead of hardcoded paths or python-config

* also fix following QA warning:
WARNING: pidgin-2.10.12-r0 do_package: QA Issue: pidgin: Files/directories were installed but not shipped in any package:
  /usr/share/appdata
  /usr/share/appdata/pidgin.appdata.xml
Please set FILES such that these items are packaged. Alternatively if they are unneeded, avoid installing them or delete them within do_install.
pidgin: 2 installed and not shipped files. [installed-vs-shipped]

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>