git-gui (Windows): Switch to relative discovery of oguilib

Instead of using an absolute path, git-gui can discover its
gui library using a relative path from execdir.  We want to
use the relative path discovery on MinGW to avoid issues
with translation of absolute paths.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Makefile

@@ -160,6 +160,7 @@ endif
 ifneq (,$(findstring MINGW,$(uname_S)))
 	NO_MSGFMT=1
 	GITGUI_WINDOWS_WRAPPER := YesPlease
+	GITGUI_RELATIVE := 1
 endif
 
 ifdef GITGUI_MACOSXAPP