rework GDEF_OS macros

Rework GDEF_OS macros:

• introduce GDEF_OS_XDG for OS that does ~/.local/share/ paths (Linux, BSD)
• we know that not only MacOS but also BSD can use the Linux pthread code
• define strlwr as strlower outside of windows in any case instead of sometime doing that define, sometime defining an NIH function, sometime defining another NIH function…
• test for !GDEF_OS_WINDOWS instead of testing for GDEF_OS_LINUX || GDEF_OS_MACOS when code is obviously done for anything that is not Windows, so BSD works without having to modify lot of lines…

Add an extra commit to fix gamepack-manager when called by legacy Makefile, this fix is required to test the changes in such scenario.

This MR obsoletes !83 (closed) (previous GDEF_OS rework attempt) and will make !75 (merged) (FreeBSD support) smaller.

Note that prior to those changes the legacy Makefile looked to not be able to build q2map on other systems than windows. It's now fixed.