Linux build improvements

[bbbbbr]

A couple changes for MakefileCommon to work better by default on Linux (it doesn't "out of the box" right now). No worries if you don't want to merge them. :)

These * have not * been tested on Windows yet since my Win VM needs a little work at the moment, so testing them would be a good idea.

• Only set GBDK_HOME if not already set, allows for local override if present.

• If GBDK_HOME is not set, and the OS is not windows, then ENV binaries are being used and a .exe extension is required for WINE. It seems an explicit path is also required by WINE, so this adds that since it is known.

• Fix zgb.lib creation for Linux. The current method was producing a line at the start of it with -e \n which resulted in the following error: ?ASlink-Warning-Cannot open library module ../Release/zgb/-e .rel If the -e was removed it still errored with ?ASlink-Warning-Cannot open library module ../Release/zgb/ .rel