Fix for compiling Ruby with VS2010 express compiler

Hi,

The first time I compiled ruby using the VS2010 express compiler for
the first time and I got an error relating to the
miniruby.exe.manifest file not being found. I changed the LDFLAGS in
win32\Makefile.sub from:

LDFLAGS = -incremental:no -debug -opt:ref -opt:icf

to

LDFLAGS = -manifest -incremental:no -debug -opt:ref -opt:icf

And it works now.

All in all, pretty good experience compiling ruby on windows now,
compared to most other packages I've tried to compile.