Help compilation in release directories.

Something about a long LDFLAGS string causes the shell to barf when
invoking gcc so that the shell crashes. This isn't a problem when
the root path is /c/gcdev, but it is when the path is
/c/gcdev/gnucash-2.6.x. Leaving off -L/mingw/lib shortens the string just
enough that it's able to run without crashing.

-L/mingw/lib isn't necessary anyway, since it's part of the system link
path.