scons: don't ignore PKG_CONFIG_LIBDIR environment variable

This is an attempt to fixup our scons pkg-config code, following
issue raised here:

https://github.com/Homebrew/homebrew-games/pull/165#issuecomment-61391362
scons: add OS_ENV option, to forward all of OS environment to scons
Merge branch 'master' of https://github.com/wesnoth/wesnoth